bitcoin: Depend on and export hex-conservative

[tcharding]

When we did the rust-bitcoin v0.30.0 release we had not yet released the hex-conservative crate so we forced users to reach into bitcoin-internals to get at hex stuff e.g., DisplayHex.

To ease the upgrade path for any remaining upgraders add a dependency on the now released hex-conservative crate and re-export it from the crate root as hex (bit late I know, sorry).

Idea came from: #2482 (comment)

We should have done this soon as we released hex-conservative.

[tcharding]

CI failures include:

• clippy
• pinning

[apoelstra]

Innnteresting. Maybe we need to pin stable same as we pin nightly. (IMO we can just drop beta, I'm not even sure what winds up on that channel but I can't imagine it's meaningfully different from nightly.)

[apoelstra]

I think a new stable was released last night https://lwn.net/Articles/966205/

[tcharding]

Maybe we need to pin stable same as we pin nightly

Isn't the problem that back then we were running clippy with stable toolchain, which we don't do now so we can just ignore these warnings?

[apoelstra]

Oh, derp, I did not realize this was a backport. Yes, in that case it's just because we used to be running clippy on stable.

[apoelstra]

utACK 8273b80 -- tested cargo test at least

[apoelstra]

It's kinda a pain in the ass to test 0.30.0, because we have the unstable docsrs feature so --all-features doesn't work, we don't have the committed lockfiles so I need to generate my own, etc.

But this is a short, obviously-correct diff so I just did some basic checks.

[tcharding]

Gentle bump, when you get around to it please.

[apoelstra]

Cool, 2 weeks (actually much longer) and no other comments. Will merge.