Make a slab allocator for general string allocation

[Ed94]

All strings that are not cached for AST name or content fields are just dumped into the global allocator.
The global allocator itself is just array of arenas (buckets). There is no reuse of any allocated memory there.

Slab classes (for 100 mb arena)

1KB   slab: 5MB  ( 5MB  / 1KB   ~ 5,000 blocks )
4KB   slab: 10MB ( 10MB / 4KB   ~ 2,500 blocks )
16KB  slab: 15MB ( 15MB / 16KB  ~ 960 blocks   )
64KB  slab: 15MB ( 15MB / 64KB  ~ 240 blocks   )
256KB slab: 20MB ( 20MB / 256KB ~ 80 blocks    )
512KB slab: 20MB ( 20MB / 512KB ~ 40 blocks    )
1MB   slab: 15MB ( 15MB / 1MB   ~ 15 blocks    )