Allow linking at the AST level

[bit-hack]

Allow multiple files to be compiled and have them generate nodes into the same AST graph. This way we can support programs broken up into smaller files/modules.

[bit-hack]

I think the API should be something like this at a high high level

ccml inst;
inst.add_source("prog", len);
inst.add_source("prog", len);
inst.compile();
....