Recall that universal notifiers are used to report changes to universal
variables to other shell instances. This adds a new strategy based on using
inotify to directly monitor the universal variables
file.
We have tried this in the past and abandoned it because it doesn't properly
work on some CI systems - let's try again.
In CMake this used a `version` file in the CARGO_MANIFEST_DIR, but
relying on that is problematic due to change-detection, as if we add
`cargo-rerun-if-changed:version`, cargo would rerun every time if the file does
not exist, since cargo would expect the file to be generated by the
build-script. We could generate it, but that relies on the output of `git
describe`, whose dependencies we can only limit to anything in the
`.git`-folder, again causing unnecessary build-script runs.
Instead, this reads the `FISH_BUILD_VERSION`-env-variable at compile time
instead of the `version`-file, and falls back to calling git-describe through
the `git_version`-proc-macro. We thus do not need to deal with extraneous
build-script running.
- This allows running `cargo fmt/clippy/test/etc` from root
- Ideally the root should be the fish-rust package instead of being virtual, but
that requires changed to CMake/Corrosion. This change should instead be
completely compatible with our existing setup.
- This also means we will only have on `Cargo.lock` for all current and future
crates.
