Use an index to find LBlocks of a particular size

[PaulBone]

Speed up allocation in the GC by adding a free list containing LBlocks of particular cell sizes that have some free space.

This was part of #180 but is now separate as its an optimisation. And I'd like to improve the benchmarks we have before working on optimisations. (Yes, it's an optimisation that turns linear into constant time).

Depends on: #180

[PaulBone]

The WIP code is here https://github.com/PaulBone/plasma/tree/gc_freelists