wasmtime: Implement table.fill

[fitzgen]

Part of #929

[alexcrichton]

Could you add a fill method to the wasmtime crate too? Also how come there are two builtin functions indices but only one actual function?

[fitzgen]

Could you add a fill method to the wasmtime crate too?

I'd prefer to make a separate PR for that, and I already have a separate checkbox for bubbling these things out to the API in the tracking issue so it shouldn't get lost or forgotten about.

Also how come there are two builtin functions indices but only one actual function?

This is the same as our table.grow implementation: they methods have the same Rust signature and essentially do the same thing regardless which reference type they are working with but Cranelift needs to have different signatures for the different variants so that it can produce stack maps for values flowing into the externref variants. We don't have much to gain by making different copies of the functions because they are already out-of-line, relatively expensive calls doing expensive operations, and we can have less code (in terms of binary size and maintenance) by having one function.