Add process1.{dropLast, dropLastIf} combinators

This PR adds the `skipLast` and `skipLastIf` combinators. `skipLast` emits every input element but the last and `skipLastIf` only skips the last element if the given predicate is true for this element. `skipLast` is equivalent to `skipLastIf(_ => true)` and `skipLastIf(_ => false)` is equivalent to `id`. The implementation of `skipLastIf` is similar to `last`.