Catch C++ codegen up and make it easy to use

[jeaye]

As part of the interop work, I added the C++ code generator back in. It compiles now, but we need to catch it back up with all of the interop expressions and we need to add allow the user to choose which codegen to do.

• Add a --codegen <ir,cpp> flag, which defaults to ir; this can be used at any time, not just for AOT
• Fix any existing compilation issues with clojure.core
• Add codegen flag to binary version
• Implement the new expressions (case*, letfn*, and all interop)
• Add CLI flag parsing to test runner
• Update bash test suite to run build/jank-test with both codegen types
• Default to C++ codegen for clojure.core

[jeaye]

Done in #626