Bump Rust toolchain version to recent nightly.

Toolchain had been frozen since 27 Jul (and the _version_ was older than
that). This gets us, among other things, the proposed subset of const
generics that is barreling toward stabilization.

As with any toolchain update, if you're using rustup, your next build
will pull down the right version automatically. However, that will not
automatically pull down our _targets._ To build again you may need to
run the following:

$ rustup target add thumbv8m.main-none-eabihf thumbv7em-none-eabihf

Author: cbiffle

rust-toolchain

@@ -1 +1 @@
-nightly-2020-06-10
+nightly-2020-12-07