PatchLang: More expressions

[woodruffw]

Some other expressions we could consider:

• Function(): Like Variable(). The constructor could take additional concretization constraints, like an optional return type or parameter types.

• Call(Function(), Variable(), ..., Variable()) and/or Call(Lit("..."), Variable(), ..., Variable()).

• Asmt(Lhs, Expression()) for lhs = <expression>;. Lhs can be either a preexisting variable or a fresh variable; we should think about ways to express the latter.