Support slicing an expression

[leonardt]

This enhances the slice node to take an arbitrary expression (rather than just a simple identifier), for now it defaults to wrapping the expression in parens except for simple cases.

[rsetaluri]

Looks good, but question: can all expressions be sliced? Not familiar with formal AST spec and what all an expression can be.

[leonardt]

I think that would be a type checker issue, e.g. generally an expression of the from (...)[4:0] would be valid syntactically, but depending on the contents of (...) it may or may not be a valid program. For our purposes, I think it greatly simplifies things to just support any expression, and require that the code generator generate a valid expression (this is easier than requiring than any arbitrary input user code is valid, since the coreir compiler generates rather simple legal expressions).

[rsetaluri]

Ok, that makes sense to me (respecting the syntactic validity but type checker would do the rest)