Adding multi-crate support through cell map

[dkcumming]

Work In Progress

Support for multi-crate Rust programs. This PR uses a cell map to store the crates. Another option would be to join all of the Items into a giant mono-crate. There is potential that using cell maps may be challenging for symbolic execution, and we should consider the other approach.

This PR:

• Changes Pgm to Crate
• Parsing and Interpreting now can take multiple .smir.json files as input
• Files are parsed into a K List
• When interpretting, that list is processed to populate a cell map of each crate
• TODO: mkFunctions
• TODO: Multi crate call
• TODO: Examples / tests

[jberthold]

This approach is now obsolete since we are linking the SMIR JSON ahead of execution