values.rkt: hash-map
env.rkt: add masks
add commented-out module stuff
remove masks
add TODO for define-extension-point, define-extension
env.rkt: change decls to not have parseExt, add results, rearrange code
rename parse -> pcomb, lang -> parse
values-ify lex.rkt
rename fmap to pmap, add pmap-maybe & rewrite pfilter in terms of it
more functionality in lex.rkt