Realloc API

[qinsoon]

Currently our bindings are implementing realloc by doing a new allocation and mem copy. I think we should introduce a realloc API. Allocators can do more efficient realloc when they can resize/expand the allocation. If they can't, they can fall back to the strategy of doing new allocation and mem copy.

[qinsoon]

We need to be clear that we do not free/collect old objects (unlike libc's realloc). We probably can use a different name such as resize instead of realloc to avoid this confusion.

An allocator can implement its realloc as an optimisation if the optimization does not slow down the normal allocation (as realloc is rare, and only used for certain bindings).

The following allocators may implement a more efficient realloc:

• bump pointer: if the object that needs to be resized is what the cursor points to, we can resize it.
• free list: if there is enough space in the cell, we can resize it.
• LOS: if there is enough space in the page, we can resize it.