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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.