Force use of nightly with rust-toolchain.toml

[morr0ne]

This pr adds a rust-toolchain.toml file that ensures the project is compiled using rust nightly.

[morr0ne]

The ci was failing because rustfmt was missing, I added it to the toolchain components. Everything should pass now

[morr0ne]

I am not sure why it's failing, maybe it wants every component to be specified?

[sunfishcode]

I'm not sure. Does it need a targets = declaration, to declare the set of targets we test?

[morr0ne]

I think I may have found the issue, the ci manually adds rust-src as a component. When it runs the build then it already has a toolchain and just install rustfmt. When its time to build it only finds rustfmt and rust-src but no core or std. The solution i think is to specify all components, including rust-src and remove that step from the ci

[morr0ne]

As I suspected everything now works. However it is using a pinned version of nightly in both rust-toolchain.toml and the ci. I don't think that's a good idea as they would need to be kept in sync. I think a better approach would be to just pin it in the rust-toolchain.toml fail and let the ci pick the version from there.

[morr0ne]

Looking more closely at the ci I feel like it could be refactored further but that is def the place for another pr. This can be merged as is.

[sunfishcode]

Sounds good.