This avoids unnecessary cache invalidations for programs not affected by
the stage0 version (which is everything except the stage0 compiler
itself).
The redundant invalidations weren't noticed until now because they only
showed up on stage0 bumps, at which point people are used to rebuilding
everything anyway. I noticed it because I wasn't adding `self.date` to
the stamp file (because I didn't realize it was necessary). Rather than
adding self.date I thought it was better to remove it from the cache
key.
This commit includes the following:
* Fix syntax errors in Python 3
* Include more docstrings in classes, methods, and functions
* Include unit tests using `unittest`
* Merge implementation of `{rustc,cargo}_out_of_date`
* Merge implementation of `RustBuild.{cargo,rustc}`
* Remove unnecessary source code
* Move all the attributes defined outside of `__init__`
* Remove remaining `%s` from print function
* Remove `WindowsError` reference on non-windows systems
* Rename some variables to be more explicit avoid their meaning
* Run bootstrap tests in the CI process
* Remove non-pythonic getters
* Remove duplicate code in `download_stage0` method
* Reduce the number of branches in `build_bootstrap` method
* Re-raise exception when we cannot execute uname in non-windows systems
* Avoid long lines
