lib/parse-sexp: apply macros

Macros implement lazy evaluation and lexical scope.  The former is
needed to make certain natural constructs work sensibly (e.g. (tag
,param)) but the latter is mainly future-proofing in case the DSL is
is extended to allow local bindings.

For technical background, see chapters 6 and 17 of [1] (or some other
intermediate programming languages textbook).

[1] http://cs.brown.edu/courses/cs173/2012/book/