Rust codegen

This is a first-pass version of Rust support for codegen.

Currently this only implements some very basic capabilities, but it does work for embedded files and it does work for test steps both with mutable and immutable contexts.

This works enough that the subplotlib.md tests are run as part of ./check

When merged, issues should opened for:

• Supporting a data dir
• Supporting running subprocesses
• A more thorough test suite as a result of the above being available

When finished, this will fix #101 (closed)