Add call/cc support

[syeerzy]

Add incomplete call/cc support . (support escape procedures only, continuation called outside dynamic extent not supported)

[frankshearar]

If you built this with the shift/reset control operators (i.e., as delimited continuations) you'd be able to trivially simulate call/cc by wrapping the entire process in a reset marker, and then also avoid all the bad things that come with call/cc.