yet more cleanups of the PERL_OBJECT, MULTIPLICITY and USE_THREADS

yet more cleanups of the PERL_OBJECT, MULTIPLICITY and USE_THREADS
builds; passing the implicit context is unified among the three
flavors; PERL_IMPLICIT_CONTEXT is auto-enabled under all three
flavors (see the top of perl.h) for testing; all varargs functions
foo() have a va_list-taking variant vfoo() for generating the
context-free versions; the PERL_OBJECT build should now be
hyper-compatible with CPAN extensions (C++ is totally out of
the picture)

result has only been tested on Windows

TODO: write docs on the THX rationale and idiomatic usage of
the Perl API

p4raw-id: //depot/perl@3667