remove unneeded env from LetCont

this is a very specific case of a more general optimization I am
imagining. Currently we keep  a lot of envs around even though we'll
never use them again later. This is particularly bad if things escape.
per hint counter in gensym
Merge branch 'master' into env-pruning
Merge branch 'master' into env-pruning
type-specialize inlined lists of sizes 1 and 2 for fixnums

use this for ConsEnv and LetCont

manually copy-pasted, didn't seem worth to write magic for these few
cases
remove leftover
add missing ack.rkt
remove debug prints
Merge branch 'master' of github.com:samth/pycket