compact_pool_revamp branch is ready for testing.

[Vasily Chekalkin]

Hello.

compact_pool_revamp branch if ready for testing. There is few compiler
warnings left but I'll fix them before merge anyway.

Main changes:
  - Allocated buffers/strings keep pointer to Memory_Block from which
they were allocated.
  - It used to skip almost full blocks during compacting.

Parrot will probably consume little bit more memory due not 100%
accuracy in calculating free space. But at least it's about 2.5%
faster on examples/benchmarks/stress_strings.pir (introduced in
branch).

-- 
Bacek