Added `iso_date` and `channel` to `rust_toolchain`. (#3957)

This change is a slight deviation from the repository interface but that
is subject to change whenever WORKSPACE support is dropped. Instead of
`version` representing both a stable version or a nightly/beta channel
with ISO date, version always represents the reported version of `rustc`
which even nightly/beta toolchains will have. Users who care about
nightly behavior should use the `channel` attribute and can make further
inferences by `iso_date` if they need to know if they are using a
version before or after a particular commit.