* The expression printer now knows how to use `let`, multi-argument lambdas and applications, and block arguments when appropriate.
* There is a separate type, AbstractSyntax, which separates parsing/printing logic from removing/reintroducing the more advanced syntax described above.
* Expression is now its own module because its 'show' depends on AbstractSyntax,
and I don't want the ast2expr/expr2ast stuff to be in the same module as the real lambda calculus stuff.
James T. Martin