The current select function only supports round robin. To change that
would be a breaking change, so for now I propose a `select_bias` which
takes a closure that defines the selection strategy.

- When a breaking change is planned, we could re-consider this, but it
  does have 4 generic parameters of which one is a closure, compared to
  the 2 streams in `select`.
- note the name select_bias. It's not consistent with `select_biased` used
  elsewhere in the lib. This can obviously be changed, but I feel the "ed"
  at the end does not add any semantic value but makes function names
  longer.
- There isn't really integration tests included right now, but the examples
  in the docs get tested by doc test.

…th_strategy`.

This allows not to print the closure and give `Select` a derive impl.

The result for `Select` looks like:

`Select { inner: SelectWithStrategy { stream1: Fuse { stream: Repeat { item: 1 }, done: false }, stream2: Fuse { stream: Repeat { item: 2 }, done: false } } }`