Remove `P: PoolAllocator` bounds on pools

[v0x0g]

Summary of Changes

• Move the bounds to the implementations, not the struct
• Non-breaking change
• Makes it nicer to use the structs in other places now.

Reasoning

This makes the pool object a bit nicer to use, since the P: PoolAllocator<T> bound is no longer present in the struct definition, so it removes the restrictions when using it as a field inside another struct.