Make functions and methods exportable and externally callable

This basically amounts to adding support for compiling libraries as opposed to just binaries. This turns out to be pretty hard for a few reasons:

we need a proc macro that produces CFFI-friendly wrapper code. I did this.
it requires updating how programs are built. I did this.
the athena target doesn't support dylib. We have to use staticlib. For some reason, the same program (with all code paths covered from main) that compiles to a ~100kb binary compiles to 5MB when it's a static lib. I haven't figured out exactly what's going on here.
it will require changing how programs are loaded and stored. We'll need at least a partial symbol table, and we'll probably need to filter for only the symbols we care about. It'll need to be possible to pass in a symbol along with a program to run.
we need to figure out a way to support multiple VM entrypoints.

athenavm / athena