accept() checks strings without copying them

json::accept() parses through json_sax_acceptor, which ignores the
values of strings and object keys, but the lexer still copied every
string token into its token buffer. That copy was 18 % of the
instructions of json::accept() on twitter.json (x86, GCC 16).

The lexer's flag for accept(), renamed from discard_number_values to
discard_values, now also selects a second instantiation of
scan_string() that performs the same checks (escapes, surrogates,
UTF-8, control characters, with the same error messages) without
writing to the token buffer. scan() chooses the instantiation once per
string: a run-time check inside the string loop made number-heavy
files up to 3 % slower on Apple M1.

json::accept() over 19 files, time after/before (geomean): 0.876 on x86
with GCC 16.2, 0.884 with Clang 23, 0.932 on Apple M1; up to 0.74 on
single files; files with few strings (canada, mesh, numbers) unchanged.

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

include/nlohmann/detail/input/parser.hpp

@@ -89,9 +89,9 @@ class parser
                     const bool allow_exceptions_ = true,
                     const bool ignore_comments = false,
                     const bool ignore_trailing_commas_ = false,
-                    const bool discard_number_values_ = false)
+                    const bool discard_values_ = false)
         : callback(std::move(cb))
-        , m_lexer(std::move(adapter), ignore_comments, discard_number_values_)
+        , m_lexer(std::move(adapter), ignore_comments, discard_values_)
         , allow_exceptions(allow_exceptions_)
         , ignore_trailing_commas(ignore_trailing_commas_)
     {