Fix clang-tidy findings in documentation examples and replace redundant aliases

Apply clang-tidy fixes to the documentation examples:
- performance-avoid-endl: std::endl -> '\n'
- readability-qualified-auto: auto -> auto* / const auto*
- performance-for-range-copy: copy -> const reference
- google-readability-casting: C-style cast -> static_cast

Replace redundant aliases (readability-redundant-qualified-alias) such as
"using json = nlohmann::json;" with "using nlohmann::json;" in the
documentation, README, examples, tests, and benchmarks. Aliases at class
scope and aliases of other types (with_base_class_t etc.) are kept.

No changes to the library or its public API. All changed examples were
compiled and run; their output matches the stored .output files (except
max_size and meta, which depend on the platform and compiler).

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

README.md

@@ -151,7 +151,7 @@ The `json` class provides an API for manipulating a JSON value. To create a `jso
 ```cpp
 #include <fstream>
 #include <nlohmann/json.hpp>
-using json = nlohmann::json;
+using nlohmann::json;
 
 // ...
 
@@ -164,7 +164,7 @@ If using modules (enabled with `NLOHMANN_JSON_BUILD_MODULES`), this example beco
 import std;
 import nlohmann.json;
 
-using json = nlohmann::json;
+using nlohmann::json;
 
 // ...
 
@@ -842,7 +842,7 @@ assert(p == p2);
 To make this work with one of your types, you only need to provide two functions:
 
 ```cpp
-using json = nlohmann::json;
+using nlohmann::json;
 
 namespace ns {
     void to_json(json& j, const person& p) {
@@ -1257,7 +1257,7 @@ See the page [quality assurance](https://json.nlohmann.me/community/quality_assu
 #include <nlohmann/json.hpp>
 
 // for convenience
-using json = nlohmann::json;
+using nlohmann::json;
 ```
 
 to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., `-std=c++11` for GCC and Clang).