In #3684 we discussed (among other things) possible syntax for operations associated with a new list container type. The front-runner for popping elements from either the front or the back of a list is using delete (delete L[0] for the front and delete L[-1] for the back). Since a very common operation would be pop-list-element-and-do-something-with-it, it would be helpful if one could write elem = delete L[0], for example, to pop the front of L and return it in elem. However, to do that delete needs to be an AST expression node. Currently, it's an AST statement node, and those don't have values associated with them.

This PR lays the groundwork for such a change. It changes delete AST nodes to be expressions that (potentially) return values. For symmetry, because it would be a bit weird to do otherwise, it also likewise changes add AST nodes.

The question is then what values should these expressions return? This PR kicks that can down the road: they currently behave as "void" types that don't return anything.

While the change is conceptually simple (and backward compatible), it winds up moving a bunch of code from various Stmt files into Expr files, along with small accompanying tweaks, so quite a bit gets touched even though nothing deep is going on.

I've also included two other minor changes. One of these is a bug fix to prevent bad initializers from leading to crashes in some circumstances. The other is to change expressions that render as words to include a space separating that word from their operand. This last (isolated to a separate commit) leads to a bunch of BTest changes that look voluminous but are in fact trivial.