[pcc] inline pcc_add_invocant and its attr accessor
[pmc2c] Faster CallContext ATTR accessors + non-static for GH #1083

Omit the obj check in the CallContext ATTR accessors.

Put CallContext non-static decl into .h as they are not static anymore,
so that we can call them now directly internally in src/call/*.c
to avoid VTABLE redirection for those.

work together with Reini Urban
[pcc] use the direct CallContext methods GH #1083

and do not use the CallContext vtable methods anymore.
also inline one Continuation_set_pointer call.

in fill_params() it is safe to use CallContext directly, as
it assumes either NULL or CallContext, but never PMCNULL.

all tests still passing, 1.63% faster.

Signed-off-by: Reini Urban <rurban@cpanel.net>
ChangeLog for GH #1083
ChangeLog for GSOC final evaluation
Merge branch 'pcc-gh1083'

Tests on all machines pass, and on most machines it's a tiny perf. win.
Release 6.7.0 - Grey-headed Lovebird

This release is still very stable. It adds two code changes:
- find_codepoint: added more name aliases for perl6 (LF,FF,CR and NEL)
- Optimize internal pcc CallContext calls from VTABLE to direct calls
  and inline CallContext ATTR accessors to omit the obj check. [GH #1083]
fresh ChangeLog, release.json for the next release