[PATCH] Small PMC (SPMC)

[Leopold Toetsch]

Here is a patch implementing the proposed scheme described in:
"[RfC] A New PMC Layout"

The speed increase in examples/benchmark/stress2?.pasm is about 1/3.

Some notes:
- all tests pass (yeah, also the property tests, we are appending the
properties beyond the SPMC in the pool arena ;-) - this needs of course
fixing)
- methods like set_same or is_equal, which access ->data unconditionally
are unused, so no breakage here AFAIK.
- extending a SPMC to an EPMC for appending properties and such is still
missing.
- the pmc_new_init() function is intended to create a normal PerlScalar
PMC, if its known in advance that e.g. the scalar has properties.

Benchmarks on different $arch welcome and of course comments.

leo