Add select_biased! macro

[ryoqun]

(this is a draft; there's too much of code duplication...)

much like rust-lang/futures-rs#1976, i want biased select in crossbeam-channel as well. :)

[ryoqun]

@taiki-e

thanks for maintaining this crate. :)

i'll group my gentle reminders with this single comment. but, I'd want some initial feedback for following draft prs. if there's some hope for merging them, I'll finish off these to cross the line, very happily:

• Add receiver.disconnect() for atomic graceful shutdown #959
• Support faster send/recv by not notifying the other side #1039
• Add select_biased! macro #1040 (this pr)

[ryoqun]

@taiki-e hey, thanks for active crossbeam maintenance. could you give this pr a initial assessment whether this pr can be merge or not ever? like other my prs, I'd like all of them to be merged eventually in some way...

thanks in advance.