A deleter field (function pointer) for CpuStorageObj

[darkestpigeon]

What

Added a deleter field to CpuStorageObj, which will be called on memalloc field instead of deallocShared if not nil.

Why

This change allows the implementation of memory sharing with other objects. For example, we can acquire memory from some opaque type

let x: SomeOpaquePointer = funcFromC(...)
var t = newTensorUninit[float]()

t.storage.cpuStorageFromBuffer(
  getData(x), cast[pointer](x),
  proc(x: pointer) {.noconv.} = let x = cast[SomeOpaquePointer](x); destroyFromC(x))

or we can wrap the tensor's memory into a shared pointer, and make deleter to delete a single copy of this pointer. This will facilitate sharing memory owned by tensors between threads.

How

A field deleter: proc(memalloc: pointer) {.noconv, gcsafe, raises: [].} was added to the CpuStorageObj. This field is then used in the destructor.