Address the review of editable json documents

From gregmarr's review of #5630:

- set() with a JSON pointer below a null value checks for "-" before
  scanning the token for digits.
- node_flags::storage is defined as escaped | edited, and the comment on
  document_data::base says that the fourth slot is unused.
- source_extent() names its root check.
- The integer-index overloads of set(), insert(), and erase() use a named
  trait instead of a negated condition, which astyle formats like the
  library's other enable_ifs.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/document_data.hpp

@@ -54,7 +54,7 @@ struct document_data
     std::vector<object_index> indexes;
     std::vector<std::uint32_t> index_slots;
     std::vector<std::uint32_t> large_objects; ///< positions of the objects to index (noted while parsing)
-    std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena, edit arena (indexed by flags & node_flags::storage)
+    std::array<const char*, 4> base = {{nullptr, nullptr, nullptr, nullptr}}; ///< string bases: source, arena, edit arena, unused (indexed by flags & node_flags::storage)
     bool discarded = true;
 
     /// The storage of edits (editable documents only; see edit_storage.hpp).

include/nlohmann/detail/view/node.hpp

@@ -34,7 +34,7 @@ struct node_flags
 {
     static constexpr std::uint8_t escaped = 1; ///< string payload lives in the decode arena, not the source
     static constexpr std::uint8_t edited = 2;  ///< string or number token lives in the edit arena (editable documents)
-    static constexpr std::uint8_t storage = 3; ///< mask: where a string or number token lives (index into document_data::base)
+    static constexpr std::uint8_t storage = escaped | edited; ///< mask: where a string or number token lives (index into document_data::base)
     static constexpr std::uint8_t is_true = 4; ///< boolean value
     static constexpr std::uint8_t moved = 8;   ///< array/object: the elements live in a separate sequence (editable documents)
     static constexpr std::uint8_t is_new = 16; ///< written by an edit: no source position

include/nlohmann/detail/view/value.hpp

@@ -114,6 +114,10 @@ struct is_non_bool_arithmetic : conjunction<std::is_arithmetic<T>, negation<std:
 template<typename T>
 struct is_non_c_string_default : negation<std::is_same<typename std::decay<T>::type, const char*>> {};
 
+/// the integral types accepted as an array index by the edits of a document
+template<typename T>
+struct is_non_bool_integral : conjunction<std::is_integral<T>, negation<std::is_same<T, bool>>> {};
+
 /// whether View::get<T>() converts to T without materialize(); get_to()
 /// assigns these, and passes the others to from_json
 template<typename T, typename BasicJsonType, typename View>