XR version 0.1.0

This is the first version of the plugin. It includes:

• Rust environment support.
• The xr function: the plugin's main interface.
• Download the latest iteration of a student when providing a UUID to one of the following sub-commands to xr:
  • test: compile and run the unit tests; this is the default when providing only a UUID.
  • bench: import a custom benchmark file from the benches directory of the plugin to the local exercise's, compile and run the contained benchmarks.
• Other xr sub-commands:
  • help: displays a description of the plugin.
  • notes: prints the mentoring notes contained in a file of the plugin's notes directory corresponding to the current exercise, or any exercise or topic when specifying a second argument.
• Default set of notes and benchmarks generously provided by @bobahop.