Extract select from src/channel.cr

[straight-shoota]

select is an independent feature and not directly related to Channel (although sending/receiving from Channel is its main purpose).
This patch extracts the implementation of select actions from channel.cr into separate files.
It's a purely organizational refactor and does not involve any semantic changes. Everything stays in the Channel namespace.

This is a preparatory step to make it easier to work on the code by separating concerns.

[ysbaddaden]

Nice, it helps to focus on each concern 👍