Switches the compiled function table from HashMap<u32, ...> to a Vector
indexed by function index. The keys are densely packed up to
functions.size() so the hash probe wasn't buying us anything.
Also drops the bounds check from MemoryInstance::unsafe_get to match
FunctionInstance::unsafe_get; the caller has already proved the address
is in range by the time we get here.
8e8a8f9270