no_std gating - gate-in alloc

[pinkforest]

There are feature gates for std that could be under alloc perhaps ? e.g.

#[cfg(not(feature = "std"))]
use alloc::boxed::Box;
use alloc::format;
use alloc::sync::Arc;
use core::marker::PhantomData;

Trouble is current rustls largely needs alloc - nonetheless

We could have all alloc gated as feature as there is talk of having Arc-less version that doesn't require Alloc ?

[newpavlov]

Yes, these should be under the alloc feature. We could leave stub like this for the time being:

#[cfg(not(feature = "alloc"))]
compile_error!("Rustls currently does not support alloc-less environments");

[Taowyoo]

Willing to take this.
I think the PR just need to update feature gate on those ::alloc imports.