Allow compiled functions to share memory

[samestep]

Currently every function created using compile allocates its own memory. This causes problems in applications that call compile many times, because current browsers do not consistently track the amount of allocated Wasm memory with their garbage collectors, and so out-of-memory errors can easily result even if the amount of memory needed at a given time is relatively small.

This PR addresses the issue by allowing the compile function to take in an optional memory parameter, which it will use (and grow if necessary) if provided, instead of always allocating a new memory.