Adds if-expressions, closes #436. Each arm of the if currently must be a simple expression; could be extended in a future PR to support blocks of commands that return an expression, if necessary.

Represented in the IR as another member of ir::Expr that takes a condition (ir::Prop) and two expressions. This worked pretty okay, just had to do some reshuffling of code in the monomorphize pass to be able to handle Props at the same time we handle Exprs.

One sort of weird thing happens in Discharge, where we need to convert the if's condition (a Prop) to an SExp as we are converting all of a component's Exprs, which happens before we do the rest of the component's Props. This means that we need to push things to Discharge::prop_map out of order, which means that SExpr needs to implement Default. We can't implement Default for SExpr, so I made a wrapper SExprWrapper that is either an SExpr orSExprEmpty, which lets us implement Default. This is a little wonky, but I couldn't really come up with a way around this.