Investigate implementing `table.copy` and `table.init` with a `memcpy`

[fitzgen]

#976 (comment)

I originally didn't do this in order to (a) conform with the spec in an obvious way, and (b) because I was unsure if table elements are something we can safely memcpy around, and (c) I wasn't sure that this actually matches the spec's behavior but I think it does.

(Note: we shouldn't literally, actually use memcpy directly. We would use copy_from_slice.)

spec link: https://webassembly.github.io/bulk-memory-operations/core/exec/instructions.html#exec-table-copy

[fitzgen]

Note: this also applies to our implementation of table.init.