lib: add try_take! macro, handling AlreadyUsed Options

[cpu]

In several places we represent something that could be consumed as an Option<T>. When we try to use it, we take() the option, match the result, and return rustls_result::AlreadyUsed if take() returned None.

Since this pattern is becoming more common with the use of more builder patterns (particularly in #341) this commit adds a try_take! macro that can do this repetitive work for us.

[cpu]

rustls-ffi / Check minimum versions (pull_request) Failing after 34s

Ah, it looks like several CI tasks are failing in this branch (and also in main's scheduled CI runs) due to rust-lang/rust#117693. We fixed this in Rustls w/ rustls/rustls#1582, but that fix is only present in 0.22.0-alpha.4.

I addressed the breakage in #341 with the update to alpha-4 (4f9ecfd) but fixing main will require a Rustls point release in the 0.21.x series. Maybe worthwhile? rustls-ffi probably isn't the only downstream crate that will break from this 🤔

[jsha]

A point release for rustls 0.21.x would be great. In the meantime I think we can unbreak the rustls-ffi build by pinning a specific nightly rust release rather than the latest nightly. We may also be able to move some of the affected tasks to a stable release.

[jsha]

This is great. One piece of naming feedback: The other try_ macros do non-mutating conversions. This one mutates, specifically calling take(). I think try_take! would be a better name and make it clearer what's happening.

[cpu]

I think try_take! would be a better name and make it clearer what's happening.

Good idea, renamed! The branch name has fallen out of date but I updated everything else. Thanks for the review.