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>

docs/mkdocs/mkdocs.yml

@@ -63,6 +63,7 @@ nav:
       - Binary Formats:
           - features/binary_formats/index.md
           - features/binary_formats/bjdata.md
+          - features/binary_formats/bon8.md
           - features/binary_formats/bson.md
           - features/binary_formats/cbor.md
           - features/binary_formats/messagepack.md
@@ -142,6 +143,7 @@ nav:
           - 'flatten': api/basic_json/flatten.md
           - 'format_as': api/basic_json/format_as.md
           - 'from_bjdata': api/basic_json/from_bjdata.md
+          - 'from_bon8': api/basic_json/from_bon8.md
           - 'from_bson': api/basic_json/from_bson.md
           - 'from_cbor': api/basic_json/from_cbor.md
           - 'from_msgpack': api/basic_json/from_msgpack.md
@@ -213,6 +215,7 @@ nav:
           - 'swap': api/basic_json/swap.md
           - 'std::swap<basic_json>': api/basic_json/std_swap.md
           - 'to_bjdata': api/basic_json/to_bjdata.md
+          - 'to_bon8': api/basic_json/to_bon8.md
           - 'to_bson': api/basic_json/to_bson.md
           - 'to_cbor': api/basic_json/to_cbor.md
           - 'to_msgpack': api/basic_json/to_msgpack.md
@@ -409,7 +412,7 @@ plugins:
       markdown_description: >
         JSON for Modern C++ is a C++11 header-only library implementing a JSON
         value type with an STL-like API, JSON Pointer/Patch, CBOR/MessagePack/
-        BSON/UBJSON/BJData binary format support, and a SAX-style parser interface.
+        BSON/UBJSON/BJData/BON8 binary format support, and a SAX-style parser interface.
       sections:
         Home:
           - index.md