using std.mem.Allocator.noResize in real allocators will leak memory

[npaskov]

According to current Allocator convention, calling resizeFn with new_len of 0 must release the memory block.

The noResize function has no knowledge of how to release memory, so if used as suggested, as resizeFn in allocators with no in-place resize ability, it will simply report success and leak the memory.

I would suggest providing explicit Allocator.freeFn for releasing memory, which will provide cleaner set of responsibilities for the Allocator functions and will allow to just have resizeFn==null if allocator does not support in-place resize.

Alternatively, if keeping current Allocator semantics, the noResize function should be removed or hidden, and allocators should always provide resizeFn capable of releasing their own allocated memory.