review of context_pmc3 branch

[Vasily Chekalkin]

Allison Randal wrote:
> This branch is a part-way step to a larger goal, so this is a sanity 
> check that these changes are headed toward the final goal.

Ok. I'm done with scope of current branch. Only one thing to do _before_ 
merge is fixing JIT failure (breaking point can be easily spotted by 
make codetest).

Next refactorings are:

1. Introduce Parrot_pcc_get_current_foo functions which will get current 
Context from Interp instead of passing it explicitly.

2. Reshuffle code in terms of revising src/call/context.c, 
src/pmc/context.pmc and src/gc/alloc_registers.c

-- 
Bacek.