Test cargo arguments passed by bootstrap.py

This moves a lot of code around, but the logic itself is not too terribly complicated.

- Move almost all logic in `def bootstrap` to the `RustBuild` class, to avoid mixing setting configuration with running commands
- Update various doctests to the new (more complete) RustBuild config. In particular, don't pretend that `bin_root` supports `build` being unset.
- Change `parse_args` not to use a global, to allow testing it
- Set BUILD_DIR appropriately so bootstrap.py doesn't panic because cargo isn't found

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