Do not compare an edited NaN equal to itself in json_view (#6085)

Two views compared equal when the tokens of two floats were equal, and
the same node was equal to itself. An edit writes a NaN as the token
"nan", so a view of it (or of an array holding it) was equal to itself
and to another NaN, while json(NaN) == json(NaN) and view == json are
false. The token shortcut now excludes the edited "nan" token (found by
the missing digit layout), dispatched on the document mode by a tag: a
read-only document cannot hold a NaN, and its walk is unchanged
(instruction count and cycles on canada, citm and twitter within
noise). For editable documents the check costs about 2 % instructions
on canada (cycles within noise). The same-node shortcut is kept for
read-only documents and for editable ones that were never edited; a
document with edits is walked, so comparing it with itself is linear
instead of constant.

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

docs/mkdocs/docs/api/basic_json_view/operator_eq.md

@@ -57,7 +57,9 @@ compared. Two arrays of different [`size()`](size.md) are rejected without visit
 Only a single number, boolean, or `#!cpp null` value is ever materialized into a `BasicJsonType`, to reuse its
 `operator==` — for numbers, so that values written differently in the source text but equal in value (e.g. an
 integer and a floating-point literal) still compare equal, following the same rules `BasicJsonType` does for special
-values such as `#!cpp NaN`. Constructing one of these scalars never allocates. Strings are compared directly, without
+values such as `#!cpp NaN`: a `#!cpp NaN` that an [edit](../basic_json_document/set.md) wrote is not equal to itself, so
+a view of it is not equal to itself either, nor is a view of an array or object that contains one, as for
+`BasicJsonType`. Constructing one of these scalars never allocates. Strings are compared directly, without
 allocating, either from the source text on both sides or, for overload 2, against `BasicJsonType`'s own string.
 Arrays and objects are never materialized at all; only their elements or members are visited, one pair at a time.