implement garbage collection

[mhuesch]

the Sto introduced in #35 is our "heap" - where much of rep_lang's allocated memory lives. much of this memory might go out of scope and not be needed, as evaluation of a program progresses.

in order to evaluate large programs, or ones where the amount of processed data exceeds machine RAM/swap (which is a required goal of the language), we will need to be able to collect garbage as we go.