Pool allocator shim

These functions provide an abstract interface to memory pools. Since multiple implementations of pooling may be useful within a single running image, the underlying substrate can be changed using by passing a constant (specifying an allocator) and parameters to pool_init.

By design, things are separated out so one can add a new pool substrate without editing this file. One can create, for example, static_pool.h, define a function vector and a parameter structure, and any functions wishing to use the static_pool could include it.