* Rust Cargo linter: Improve workspace support
When using Cargo workspaces [1], there is a 'Cargo.toml' directory in a
top level directory, listing all the crates in the project. If we are
currently editing one of the crates, 'cargo build' should execute in
that directory for that crate's separate `Cargo.toml`, otherwise Cargo
may spend more time possibly rebuilding the entire workspace, and maybe
failing on one of the other crates, instead of succeeding on the current.
[1] https://doc.rust-lang.org/book/second-edition/ch14-03-cargo-workspaces.html
* When working on rust/cargo projects of varying sizes, it may be useful
to either build all possible features (i.e. lint all possible
conditionally compiled code), or even turn off other features for a
quicker edit-lint cycle (e.g. for large projects with large build times)
* Added a g:ale_rust_cargo_default_feature_behavior flag for instructing
cargo to not build any features at all (via `--no-default-features`),
building default features (via no extra flags), or building all possible
features (via `--all-features`)
* Also added a g:ale_rust_cargo_include_features flag for including
arbitrary features to be checked by cargo. When coupled with
g:ale_rust_cargo_default_feature_behavior this allows for full
customization of what features are checked and which ones are ignored
