Document that read() must not be given a view of the document itself (#6086)

* Document that read() must not be given a view of the document itself

basic_json_document::read() releases the previous text, the decoded
strings and the edits before it parses its input. An input that points
into them (source(), get_string(), number_token(), or a string of an
edited document) is read after its memory was cleared or freed.

A guard that copies such input costs about 1 ns per read() (5 % of a
read of a 7-byte input), which this API exists to avoid, so state the
precondition in read.md, lifetime.md and the pages of the views that
hand out such strings instead, and show how to copy the string first.

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

* Assert in debug builds that read() is not given its own text

read() releases the document's text, decoded strings, owned image and
edits before it parses, so an input that points into them is read after
release. Without NDEBUG, read() now asserts that a borrowed input, a
copied contiguous input and (before C++17) a string_ref input are not
held by the document: its owned text, its decoded strings, an owned
image, and the current buffer of edited strings. Release builds keep
the documented precondition without a check, which would cost 2-4.5 %
per read() of a small message.

JSON_ASSERT is undefined at the end of json.hpp, so the view uses
assert() through a new internal NLOHMANN_VIEW_ASSERT.

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

---------

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

include/nlohmann/detail/view/macro_unscope.hpp

@@ -18,6 +18,7 @@
 #undef NLOHMANN_VIEW_NOCLONE
 #undef NLOHMANN_VIEW_NODISCARD
 #undef NLOHMANN_VIEW_THROW
+#undef NLOHMANN_VIEW_ASSERT
 #undef NLOHMANN_VIEW_LITTLE_ENDIAN
 #undef NLOHMANN_VIEW_REPEAT16
 #undef NLOHMANN_VIEW_REPEAT_SECOND_8