Add BON8 support

Add to_bon8/from_bon8 and input_format_t::bon8 for BON8, a binary format
that uses the byte values that cannot begin a UTF-8 character as type
markers, so strings need no length prefix. It is the most compact of the
supported binary formats on the benchmark files.

The reader is non-recursive like the other binary readers. A string ends
at the first byte that cannot continue it, so the reader hands the one or
two bytes it reads past a string back to the value that follows. The
writer produces the canonical representation of the specification, except
for NFC normalization; its output is identical to that of the reference
implementation (HikoGUI) on all files of the test data.

The round-trip tests need the .bon8 files of json_test_data 3.2.0.

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

include/nlohmann/detail/input/input_adapters.hpp

@@ -34,7 +34,7 @@ namespace detail
 {
 
 /// the supported input formats
-enum class input_format_t { json, cbor, msgpack, ubjson, bson, bjdata };
+enum class input_format_t { json, cbor, msgpack, ubjson, bson, bjdata, bon8 };
 
 ////////////////////
 // input adapters //