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

rust/README.rst

@@ -61,8 +61,8 b' time to the python interpreter instead o'
 Developing Rust
 ===============
 
-The current version of Rust in use is ``1.34.2``, because it's what Debian
-stable has. You can use ``rustup override set 1.34.2`` at the root of the repo
+The current version of Rust in use is ``1.41.1``, because it's what Debian
+stable has. You can use ``rustup override set 1.41.1`` at the root of the repo
 to make it easier on you.
 
 Go to the ``hg-cpython`` folder::

rust/hg-core/Cargo.toml

@@ -27,10 +27,8 b' memmap = "0.7.0"'
 zstd = "0.5.3"
 rust-crypto = "0.2.36"
 
-# We don't use the `miniz-oxide` backend because its minimum Rust version is
-# `1.36`. However, this PR (https://github.com/Frommi/miniz_oxide/pull/84/files)
-# introduces a flag `no_extern_crate_alloc` to bring the requirement back down
-# to `1.34`.
+# We don't use the `miniz-oxide` backend to not change rhg benchmarks and until
+# we have a clearer view of which backend is the fastest.
 [dependencies.flate2]
 version = "1.0.16"
 features = ["zlib"]