rust: update minimum supported Rust compiler version
Debian stable backported `rustc 1.41.1` for a Firefox release that needed it,
so we can finally update the compiler version to something that isn't more than
a year old.
This means that (among other things [1]), `async await` syntax is usable!
Differential Revision: https://phab.mercurial-scm.org/D9138
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.