Allow slices of more types than `u8` in wiggle

#### Feature

The `GuestMemory::as_slice` method should allow slices of types other than `u8` to be constructed. It's currently possible to work around this by using the `GuestMemory::to_vec` method, which is appropriately parameterized, but it would be nice to read slices of types like `u32` without having to copy all the values back into the host.

#### Benefit

This change woudl result in a slight memory savings for cases where you don't need the entire vector to be present in host memory.

#### Alternatives

We can certainly leave things as they are, as it's always possible to fall back on `GuestMemory::to_vec`.