Keep local Rust toolchain version in sync with CI automatically.

[greenhat]

Set the Rust toolchain version via rust-toolchain.toml and use it in 'cargo make`.
See details at https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file

[bitwalker]

I've added a rust-toolchain.toml file to the repo, and set it to the nightly toolchain until the trait_upcasting feature is stabilized in the next release, then we'll switch back to stable. In any case, it'll be easier to keep things in sync using that from now on.

Added in #50

[greenhat]

cargo-make on CI is still driven by CARGO_MAKE_TOOLCHAIN env var set in ci.yml. So in order to simulate CI on a local machine one needs to set this env var. I guess removing the env var might be enough for cargo-make to follow the rust-toolchain.toml.

[greenhat]

Done in #91