Cache build results per feature matrix for dependencies

[dhardy]

Concrete example:

git clone https://github.com/maidsafe/routing.git && cd routing
cargo build
cargo build --features=clippy
cargo build

All three build commands causes many dependencies to be rebuilt from scratch. I would expect the third command not to rebuild dependencies which had already been built with the current feature matrix; currently toml, log4rs, maidsafe_utilities, crust and routing itself get rebuilt by the last build command.

Note that when switching release and debug builds, Cargo does cache dependency builds and not force a rebuild when switching.

[alexcrichton]

Thanks for the report! This is actually the topic of #3102, which I believe should fix this

[dhardy]

Great!