Features don't interact well with workspace crates

[iliekturtles]

See https://internals.rust-lang.org/t/help-us-test-the-breaking-bug-fix-to-cargo-features/7317 and rust-lang/cargo#5364. Also see the current .travis.yml where cargo test is run once for uom and once for feature_check.

Currently if you use cargo --test --all workspace crates are compiled with their default features (--features does not affect them) and link against the root package that is built with the specified features including dev-dependencies. If you use cargo test --manifest-path ... to manually run test for a workspace crate that references the root package then the root package is recompiled without dev-dependencies.

Ideally the uom CI build scripts should run both uom and feature_check tests and only compile uom once.

A possible short term fix is to have feature_check use the same feature set in the cargo build invocation so those build artifacts can be reused.

[iliekturtles]

Currently cargo test --all --features ... still does not pass features to all workspace crates. Oddly workspace crates still use the version compiled with features. Currently feature_check is reusing the previously compiled uom but edition_check isn't because --manifest-path needs to be used as the crate is not part of the workspace. Once the minimum required rustc is bumped to 1.31.0 (Rust 2018 edition) then edition_check can be added back into the workspace and this issue should be reviewed again.