Provide API to request memory from a specific block

The library already supports multiple independent blocks of memory. In some systems it might be required to allocate certain buffers in specific locations. In my current use case I have an embedded system with various memories and a microcontroller with an a integrated USB controller. The latter interacts with the CPU via a (complicated) shared memory interface. Due to size constraints I have put the heap in an external memory (PSRAM) that is accessible to the CPU via normal load/store instructions but apparently not for the USB controller. However, the vendor library uses the heap to allocate buffers for USB transfers. `(┛ಠ_ಠ)┛彡┻━┻`

As of now, the project uses newlib and https://github.com/DRNadler/FreeRTOS_helpers/ but I might give implementing this in libmemory a shot if you think it is in scope. Any thoughts on the API very welcome. Another embedded allocator that supports this feature is https://github.com/MaJerle/lwmem