Slicing arrays, slices and bytes at compile time

[lerno]

We want to support the slicing of arrays, slices and bytes at compile time. The suggestion is that first we allow compile time slices to implicitly convert to arrays. This makes semantics better and we pass around the data properly. So:

1. Slice bytes => bytes
2. Slice array/vector => constant slice
3. Slice constant slice => constant slice
4. Slice untyped list => untyped list

[lerno]

In dev