Types::Serialiser::Boolean for JSON-XS-3.01 interop

 - Change booleans interop logic (again) for JSON-XS-3.01
   Check now for Types::Serialiser::Boolean refs (#18 clintongormley)
   to avoid allow_blessed for JSON-XS-3.01 booleans.
- add t/52_object.t from JSON::XS
- add .gitignore
3.0103: backport allow_tags, decode_tag, FREEZE/THAW callbacks from JSON-XS-3.01

- support JSON::XS extensions for OBJECT SERIALISATION: allow_tags, FREEZE/THAW
- improve encode_hv HE sort on stack < 64 or heap to avoid stack
  overflows (backported from JSON-XS-3.01). do not use alloca.
- fix t/52_object.t
- fix t/96_interop.t if JSON is not installed