HostId enum Refactor

[cbrownsey]

Original Issue: #1260

This implements the change to having HostId include a variant for all possible hosts under any compilation configuration.

This also adds an implementation of TryFrom<HostId> for Host, which seemed the best way to have that. This also keeps available_hosts, host_from_id, and ALL_HOSTS, although they no longer seem relevant to keep anymore.

[cbrownsey]

Ack! Rookie mistake. All green now, but I'm not sure if I'm entirely keen on pushing as-is. I think the HostId enum should be pulled out of the platform module and be re-exported at the crate root. Additionally, I'm thinking that the available_hosts, host_from_id and ALL_HOSTS items should be canned. But this is what I outlined in the initial issue, so that's what I'm committing right now.

[1rhino2]

static pass on the HostId refactor (no runtime concerns here).

direction looks right for #1260: a single non_exhaustive HostId with every variant, plus is_supported / is_available / available_hosts(), and TryFrom for Host. that is cleaner than cfg-gated enums for cross-platform user code.

a few things to nail before merge

• AvailableHostsIter::next currently returns hosts where is_supported() is true, but the docs on available_hosts / HostId::available_hosts talk about availability. supported != available (daemon down, etc). looks like the iterator should call is_available(), or the naming/docs need to match whichever filter you want.
• Display writes ascii-lowercase ("wasapi") while FromStr examples/docs show mixed case like "WASAPI" / "ALSA". case-insensitive parse is fine; worth one golden test that Display round-trips through FromStr.
• body notes available_hosts / host_from_id / ALL_HOSTS may be obsolete. if they stay, mark deprecated in the same PR so the migration path is obvious; if they go, UPGRADING.md needs a short note.
• needs a rebase check against current master (this sat a bit).

i can help poke windows HostId::Wasapi / Asio availability after rebase if useful.

[roderickvd]

Valid point that supported isn’t equal to available. But to your last point, in which cases would that be different for WASAPI or ASIO?

[cbrownsey]

I've made some changes. I decided to go for removing available_hosts, ALL_HOSTS, and host_from_id. I added a very short section about how to go from a HostId to a Host, which seems enough to me.

Thank you for spotting that I did use the wrong method in AvailableHostsIter, not sure what that was about. And I did remove the lowercasing in the Display method. I kept that from the previous implementation, but I never did like it as it felt like a very unnecessary allocation. I also added a test to ensure that the Display implementation round-trips, as you mentioned.

I've also updated some of the example files to have them compile with my changes, but they do still have the cfgs in them that this whole exercise is to remove, so I'll have those fixed up tomorrow. Other than that, I'm comfortable with the changes as they are.