Placement allocation / specifying an allocator
==============================================

When using arena allocation, and also to make things more efficient,
one will often want to use custom allocation when creating a new object.

So when a function directly or indirectly creates something that would
be allocated on the heap, we need to have a parameter (or a thread-local
variable) to specify the allocator.

(Also, there may be some cases where you want to always use the heap,
such as for lazy initialization and libraries that do built-in caching
etc.)


The allocator needs to contain this info at least:
- Type of allocator
- Allocator specific data, e.g. pointer to allocation area

type Allocator = case struct (
    .Heap ( )
    # FIXME it is probably better to return a non-pointer struct value in this case? (i.e. stack return)
    #.MergedImplicitSize (
    #    []byte data
    #)
    .MergedFixedSize (
        size len
        [len]byte data
    )
    .FixedSize (
        size len
        rwref [len]byte data
    )
    .Arena (
        rwref Arena arena
    )
    # if we alloc function pointers
    .CustomAllocator (
      
    )
)