Change crate package layout to reflect Rust standards

[virtualritz]

See here.

At the moment examples and tests all live under src. This also means there is a namespace prefix, core, that could potentially be omitted.

If they were moved to their own resp. folders this crate could be lighter. It may be necessary to move examples to a twsapi-examples crate. The tests could then depend on this crate.

See the bevy crate for some ideas how to set up a workspace with sub-crates that makes this more easy.

P.S.: I may do this myself for my fork and open a PR here.