First step to using rustup.

[dkcumming]

• Need build.rs so that correct rust version is used with components installed
• Need to update README.md
• Building with Makefile needs to be removed
• Handling tests with Makefile needs to change to use cargo
• Change tests to use cargo test ? Could remove Makefile entirely then
• CI needs to be updated succesfully
• Need to handle golden tests difference
• Also need to make sure this will still work with mir-semantics and extracting stable-mir-json for std lib