Allow custom memory allocator Reallocate to fail

[ncruces]

Hey! 👋

This is just an idea, but the change was small enough that I decided to just try it.

How would we feel about allowing the custom allocator to return nil to signal it can't grow the linear memory?

In Go, a failure to allocate memory is a fatal condition, so it makes sense that we don't think about this when using append to grow the linear memory.

But custom allocators can handle this, it doesn't need to be a fatal condition. Also Wasm modules can deal with it: e.g. for wasi-lib, if sbrk fails, malloc just returns NULL, and (e.g.) SQLite can deal with that by freeing caches and trying again (if configured to do so).

cc: @anuraaga

[mathetake]

Sounds reasonable to me

[ncruces]

OK. I'll add a test and document the behavior in the interface.