Add `Vec::drain` and `String::drain`.

[reitermarkus]

Closes https://github.com/rust-embedded/heapless/issues/334 and https://github.com/rust-embedded/heapless/pull/365.

[Dirbaio]

could you add tests?

[reitermarkus]

I can't quite figure out the miri error. Note that the implementation is almost 1:1 copied from std, so I'm not sure if it's a false positive.

[Dirbaio]

rebased it for you on top of the new VecView impl.

[reitermarkus]

@Dirbaio, thanks!