Release 0.160.0

Bugs fixed:

• A C++20 change caused a basic_json overloaded operator '==' to be ambiguous
  despite there being a unique best viable function. Fixed.

• When parsing MessagePack buffers, lengths were being incorrectly parsed as signed integers. Fixed.

Enhancements:

• Added jsonschema extension that implements the JSON Schema Draft 7
  specification for validating input JSON, #280

Changes:

• Until 0.160.0, jsonpointer::flatten, when applied to an
  an empty array or empty object, would produce a flattened value
  of null rather than [] or {}. Since 0.160.0, it will
  produce [] or {}. For example, given {"bar":{},"foo":[]},
  the flattened output was {"/bar":null,"/foo":null}, but is now
  {"/bar":{},"/foo":[]}. jsonpointer::unflatten will now return
  the original object.

Deprecated function removed:

• The long deprecated basic_json function
  to_string(const basic_json_encode_options<char_type>&, char_allocator_type&) const
  has been removed (replacement is dump).