Add miri.toml config file for ./miri script

[RalfJung]

As discussed in #4700, this adds a miri.toml config file for our ./miri build script. The supported config flags in this PR are:

[toolchain]
# Overwrite the default toolchain name used by `./miri toolchain`.
# Note that all other commands will just use the currently active rustup toolchain!
# (Though note that if you have `auto.toolchain` enabled, most commands will run `./miri toolchain`
# first, which will activate the toolchain given here.)
name = "miri"
# Additional components to install with the toolchain. Note that if you remove `clippy` or `rustfmt`
# from this list then obviously `./miri clippy`/`./miri fmt` will not work.
# Only takes effect when a new toolchain is installed. Run `rustup toolchain remove <name>` followed
# by `./miri toolchain` to force this to have effect.
components = ["clippy", "rustfmt"]

[auto]
# Automatically run `./miri toolchain` before most commands.
# Uses the toolchain name configured above, if any.
toolchain = false
# Automatically run `./miri clippy` before most commands.
clippy = false
# Automatically run `./miri fmt` before most commands.
fmt = false

The old .auto* files are still supported: they are used as fallback if there is no miri.toml file.

@rust-lang/miri what do you think? Any feedback on the names for stuff in the config file?

[RalfJung]

I absolutely love how I don't even have to store these in a Vec, I can just directly pass the iterator to cmd!. :D

View changes since the review

[saethlin]

@rust-lang/miri what do you think? Any feedback on the names for stuff in the config file?

I'll probably just set-and-forget. Looks good to me.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.