Implement From<Foo> for Descriptor<Pk> using macro

[tcharding]

Add a macro and remove all the individual from impls.

Refactor only, no logic changes.

[apoelstra]

I wish there was a way that you could expand macros prior to committing them. So we could have a CI job that checked that all the code was actually produced from macros, but when reading it, you could read the expanded versions.

In C you can kinda do this by running cpp directly.

In Rust I guess we could also use cpp but that's maybe too evil :).

But basically, I like the guaranteed consistency and ease of writing macros but I really think they harm readability.

[tcharding]

but I really think they harm readability

The first Rust job I had they barred me from writing macros because of this.

Perhaps we can find a middle ground, if the macro was called in a form more like what the code looks like it would be easier to read. It may even be easier to read because it reduces the noise.

Convert this

impl From<error::Bar> for GeneralError {
    fn from(e: error::Bar) -> Self { Self::Bar(e) }
}
with one of these:
```rust
impl_from_for_error_type!(From<error::Bar> is GeneralError::Bar);
impl_from_for_error_type!(From<error::Bar>, GeneralError::Bar);
impl_from_for_error_type!(error::Bar, GeneralError::Bar);

(I've use an error conversion because of rust-bitcoin/rust-bitcoin#1950 (comment))

[apoelstra]

Can you use for instead of is? Or does Rust prevent this?

[tcharding]

We can put anything we want in there I believe, I did not code the actual macro up though. (I also had for at one stage when editing the original post)

[tcharding]

oh that's right, I removed for because its a bit weird to have for and then include the variant name.

[apoelstra]

Ah, I see. So even in your hypotheticals I misread what was going on ... I think the explicit code is probably better than anything we've got here.

One thing we could experiment with is adding #[from] the way that thiserror does. We've shied away from syntax extensions in the past but possibly this one is easy to do. Unsure. It would probably depend on quote which we've had MSRV trouble with in the past.

[tcharding]

I"ll leave this for now.