Pin toolchain to specific nightly version (#66)

* Use new toolchain TOML format
  This allows us to specify a specific version and needed components that rustup can then automatically install.

rust-toolchain

-nightly
+[toolchain]
+channel = "nightly-2020-11-25"
+components = [ "rustfmt", "rust-src", "llvm-tools-preview"]
+targets = [ "x86_64-unknown-hermit" ]