Rust facilities

This MR contains a bunch of facilities for the Rust subplotlib library, along with tests for them.

Tests have been, for the most part, taken from the existing tests for runcmd and files, though they had to be copied into place due to limitations in how we handle input files for now.

This closes #140 (closed) and closes #139 (closed).

While this goes someway toward #141 (closed) it does NOT fix it yet.