Add RecoveryId getters

[tcharding]

Add two getters to the RecoveryId type

• to_u8()
• to_u32()

Original mentioned here: #765 (comment)

For anyone that wants to know why we add getters to int types and not setters, its because the type of From often cannot be inferred so the explicit getter is more ergonomic.

[apoelstra]

I don't think we need to_u32. If we have to_u8 then this can be converted to every other integer type (except i8) with .into().

If we currently have to_i32 then we can keep that but we should deprecate it.

[Kixunil]

Also as is still possible but TBF some people like to deny as in clippy to avoid problems.

A constructor for consideration though: from_u8_masked that infallibly converts x & 3.

[apoelstra]

Oh, I like that idea.

[tcharding]

Should it take a u8 or i32? In rust-bitcoin we use

            let recid = RecoveryId::from_i32(((bytes[0] - 27) & 0x03) as i32)?;

[apoelstra]

Let's say a u8. It's a little tempting to do a T: Into<u64> or something crazy but there's really no known usecase for this except that expression in rust-bitcoin.

[Kixunil]

That expression is also silly. It takes a byte, transforms it to a different byte then converts it to i32, a bigger and signed type, and then back to what's effectively u2. That useless intermediate conversion should just be removed.

[apoelstra]

Well, given the existing API it's not too silly :P. But yes, we should fix the API so we can fix it.