bdj/dybvig-three-imp

Implementations of the compilers and VMs from R. Kent Dybvig's "Three Implementation Models for Scheme." Each file is named after the section of the paper it implements. I made a few modifications to make the code work with PLT Scheme. Each file is a module that exports an evaluate function. Some of the VMs need mutable lists, so I explicitly use mcons, etc. where needed. Also, the recur macro is the same as let, so I used let instead.