Define a MSRV, use that toolchain by default.

[jrvanwhy]

This PR defines a Minimum Supported Rust Version. It specifies that toolchain in rust-toolchain, so rustup will use that toolchain by default. The test-stable make action is removed, as it is now redundant.

We still need the nightly toolchain for Miri. Instead of specifying the nightly toolchain version via the command line, I created a new nightly/rust-toolchain.toml file that specifies the toolchain. The benefit is this makes rustup automatically install the nightly toolchain including Miri. This should stop the toil we currently have where every time the Rust toolchain is updated, everyone using make test has to manually install new toolchains and Miri. The downside is the directory change we need to use it is kinda ugly.

I set the MSRV to 1.70 so this doesn't conflict with #498.

I will send another PR that adds rust-version.workspace = true to all the Cargo.toml files in this repository; I didn't want to clutter this PR with those changes.

[bradjc]

@jrvanwhy and I discussed this quite a bit, and there isn't a perfect solution (that we know of) and I think this one is as close as we can get.

What we want:

• To build libtock-rs on stable by default.
• To inform users of the minimum stable version they need to compile libtock-rs (as the lib may use APIs added in specific versions of the core library and therefore need at least a certain version of stable).
• To test in CI on exactly the minimum version specified to ensure it is in fact the minimum version.
• To be able to run miri (which is only available on the nightly toolchain).
• To allow users to easily run the same tests locally as in CI.
• To avoid unrelated clippy errors or compiler warnings causing a PR to fail CI because the PR happened to be timed such that it caused the first run of CI on a newer stable toolchain.

What would be nice:

• To allow users to use whatever version of stable they already have installed, as long as it is new enough to meet the MSRV.
• To always be using the latest clippy and compiler to find as many potential issues as early as possible.
• To require as few toolchains as possible.

Using a fixed stable version doesn't get us the first two points of "would be nice" but it gets us all of the must haves and minimizes the number of toolchains a user must have to run tests locally.