Add array slice optimizations

Currently we don't have any array slice optimizations. Here are a couple which might be useful:

1. ArraySlice of an Array operation where the ArraySlice has a literal start. In this case you should be able to replace the ArraySlice with an Array operation which gathers the statically-known element of the slice from the original Array operation's operands.
2. ArraySlice of an ArraySlice in some instances might be replaceable with a single ArraySlice.

A (potentially crazy) idea might be to decompose ArraySlice into a bunch of ArrayIndex's followed by an Array operation to gather them up, We already have lots of optimizations for ArrayIndex which would then automatically kick in. This would avoid duplicating a lot of complicated index logic. And it doesn't fundamentally affect the scaling of XLS for big arrays because we already have to emit array operations as element-by-element in Verilog.