Fuzz json_document with exact-size buffers and parse options

The fuzzer only parsed a std::string with default options. Also parse an
exact-size byte vector, which has no NUL after its last byte and takes the
bounds-checked path, and derive ignore_comments and ignore_trailing_commas
from the first input byte, comparing against json::parse and json::accept
with the same options.

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

tests/fuzzing.md

@@ -6,7 +6,10 @@ Each parser of the library (JSON, BJData, BON8, BSON, CBOR, MessagePack, and UBJ
 Additionally, `parse_json_view_fuzzer` (`tests/src/fuzzer-parse_json_view.cpp`) cross-checks `json_document`/`json_view`
 (the zero-copy, read-only view declared in `json_view.hpp`) against `basic_json` on the same JSON text: it asserts that
 `json_document::accept` agrees with `json::accept`, that an accepted input materializes to the same value `json::parse`
-produces, and that a rejected input makes both parsers throw with an identical `what()`. It takes plain JSON text, so it
+produces, and that a rejected input makes both parsers throw with an identical `what()`. It checks this for a
+`std::string` input (borrowed, with a NUL after the last byte) and for an exact-size `std::vector<std::uint8_t>`
+(borrowed, with nothing after the last byte), and for the `ignore_comments` and `ignore_trailing_commas` options, which
+are taken from the low bits of the first input byte (the byte stays part of the text). It takes plain JSON text, so it
 reuses the `corpus_json` corpus rather than a format of its own.
 
 ## What the fuzzers check