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Test that the contiguous fast paths never read outside the input (#5826)

* Test that the contiguous fast paths never read outside the input

The bulk string scanner, the SWAR searches, the UTF-8 prefix validator,
the 8-digit number loop, the 8-byte indentation skip, and the BSON/BON8
bulk string readers load several bytes at a time from contiguous input.
ASan misses over-reads into the slack of the same heap allocation, and
fuzzers rarely end an input exactly at a page boundary.

The new test copies each input directly in front of, and directly
behind, an inaccessible page (mmap/mprotect on POSIX,
VirtualAlloc/VirtualProtect on Windows), so reading even one byte
outside the input crashes. It covers the string-scan functions directly
and json::parse, accept, sax_parse (with and without comments) and all
binary readers through the public API, with inputs ending at every byte
of strings, escapes, UTF-8 sequences, numbers, literals, indentation
runs, and binary documents. Each result must match parsing the same
input from a std::string.

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

* Rename the reader parameter to avoid a flawfinder false positive

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

* Make the guard page test work without exceptions and under UBSan

Under JSON_NOEXCEPTION, the error paths abort instead of throwing, so the
outcomes are now computed with allow_exceptions=false there. Copying the
input with std::copy from char to unsigned char tripped Clang's implicit
conversion sanitizer; std::memcpy avoids the conversion.

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

* Test parse_float_native() at the guard page

parse_float_eisel_lemire() was removed by #5738, so the test no longer
compiled. The lexer converts floats from its token buffer, so the JSON
section never places a float token at the page boundary; the section now
calls parse_float_native() for float and double on every complete number
token directly and compares the result... (continued)

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