A fuzz bug was hit last night where the root of the fuzz bug appears to
be exhaustion of the virtual address space. The specific case in
question instantiated a module with ~100 memories ~100 times, and each
memory reserved ~8gb of the virtual address space. This takes around 47
bits of addressable memory which is mighty close to the limit of what
can be done on x86_64, so this commit reduces the number of memories
that an instance may have when coming out of `wasm-smith`.
e73446790e