reduce copies in LetCont.plug_reduce
just cache the stored value on the ModuleVar

no need to constantly do a full lookup during interpretation, it can't
change anyway
shortcut for making Values of size 1 (the common case)

somewhat hacky implementation
just pre-allocate the list instead of appending
shortcut the expensive use of label in this very common case
somewhat messily remove one level of indirection in label
simple primitives don't need a label to return their value

their is no potential for recursion
don't use slicing
correct ANF

implementation is quatratic in the depth of expressions for now
Merge branch 'master' of github.com:samth/pycket