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
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed