Implement support for string_view (attempt no. 3) (#3423)

* Add key_compare member to ordered_map

* Replace == with key_compare in ordered_map

* Expose the actual comparison function used by object_t

nlohmann::ordered_map uses a different comparison function than the one
provided via template parameter.
* Introduce a type trait to detect if object_t has a key_compare member.
* Rename object_comparator_t to default_object_comparator_t.
* Add object_comparator_t to be conditionally defined as
  object_t::key_compare, if available, or default_object_comparator_t
  otherwise.
* Update the documentation accordingly.

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

* Add type traits to check if a type is usable as object key

Add type trait to check:
* if a type is a specialization of a template.
* if a type is a json_pointer.
* if a type is a basic_json::{const_,}iterator.
* if two types are comparable using a given comparison functor.
* if a type is comparable to basic_json::object_t::key_type.
* if a type has a member type is_transparent.
* if a type is usable as object key.
* if a type has an erase() function accepting a given KeyType.

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

* Rework basic_json element access to accept more key types

Rework basic_json element access member functions and operators to
accept any type that meets the requirements defined by type trait
detail::is_usable_as_key_type.

Member functions and operators:
* at()
* operator[]
* value()
* erase()
* find()
* count()
* contains()

Update documentation to reflect these changes.

Add unit tests to excercise the new functions using std::string_view.

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

doc/mkdocs/docs/api/basic_json/at.md

@@ -10,13 +10,28 @@ reference at(const typename object_t::key_type& key);
 const_reference at(const typename object_t::key_type& key) const;
 
 // (3)
+template<typename KeyType>
+reference at(KeyType&& key);
+template<typename KeyType>
+const_reference at(KeyType&& key) const;
+
+// (4)
 reference at(const json_pointer& ptr);
 const_reference at(const json_pointer& ptr) const;
 ```
 
 1. Returns a reference to the array element at specified location `idx`, with bounds checking.
-2. Returns a reference to the object element at with specified key `key`, with bounds checking.
-3. Returns a reference to the element at with specified JSON pointer `ptr`, with bounds checking.
+2. Returns a reference to the object element with specified key `key`, with bounds checking.
+3. See 2. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
+4. Returns a reference to the element at specified JSON pointer `ptr`, with bounds checking.
+
+## Template parameters
+
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
 
 ## Parameters
 
@@ -25,15 +40,16 @@ const_reference at(const json_pointer& ptr) const;
 
 `key` (in)
 :   object key of the elements to access
-    
+
 `ptr` (in)
 :   JSON pointer to the desired element
-    
+
 ## Return value
 
 1. reference to the element at index `idx`
 2. reference to the element at key `key`
-3. reference to the element pointed to by `ptr`
+3. reference to the element at key `key`
+4. reference to the element pointed to by `ptr`
 
 ## Exception safety
 
@@ -51,7 +67,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
       in this case, calling `at` with a key makes no sense. See example below.
     - Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if the key `key` is not
       stored in the object; that is, `find(key) == end()`. See example below.
-3. The function can throw the following exceptions:
+3. See 2.
+4. The function can throw the following exceptions:
     - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in the passed
       JSON pointer `ptr` begins with '0'. See example below.
     - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in the passed
@@ -68,9 +85,10 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Complexity
 
-1. Constant
+1. Constant.
 2. Logarithmic in the size of the container.
-3. Constant
+3. Logarithmic in the size of the container.
+4. Logarithmic in the size of the container.
 
 ## Examples
 
@@ -134,7 +152,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
     --8<-- "examples/at__object_t_key_type_const.output"
     ```
 
-??? example "Example (3) access specified element via JSON Pointer"
+??? example "Example (4) access specified element via JSON Pointer"
 
     The example below shows how object elements can be read and written using `at()`. It also demonstrates the different
     exceptions that can be thrown.
@@ -149,7 +167,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
     --8<-- "examples/at_json_pointer.output"
     ```
 
-??? example "Example (3) access specified element via JSON Pointer"
+??? example "Example (4) access specified element via JSON Pointer"
 
     The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions
     that can be thrown.
@@ -173,4 +191,5 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 1. Added in version 1.0.0.
 2. Added in version 1.0.0.
-3. Added in version 2.0.0.
+3. Added in version 3.11.0.
+4. Added in version 2.0.0.