Add platform API for low-level allocator hooks

[adamjs]

Add support for all allocations in the library to optionally be handled by the user (instead of the CRT or OS allocators).

This allows several things:

• Full memory reclamation upon library shutdown
• Custom allocators (fine-tune for performance or memory size)
• Additional security checks (isolate memory in separate address space)

[adamjs]

This feature is now complete and will be included in 1.4.