Fix the mess around Parity

[tcharding]

Recently I opened up a hornets nest by changing the parity boolean into an opaque type (#344). This PR was also flawed, @sanket1729 came to the rescue in #360 and #361.

There has now been much talk and disagreement on this, listing links here in an attempt to collect all the comments (in no particular order):

• Use un/tweaked public key types rust-bitcoin#728 (comment)
• Update to secp256k1 0.21.2 rust-bitcoin#755 (review)
• Converting LeafVersion into an enum rust-bitcoin#718 (review)

[apoelstra]

My proposed solution:

• Redefine Parity to be an enum Even/Odd rather than an opaque type (not a breaking change)
• Add to_u8 and fallable from_u8 methods (we can bikeshed on names, I don't care) (not a breaking change)
• Add an xor method between two Paritys/overload the ^ operator (not a breaking change)
• Deprecate the existing i32 conversion methods
• Make sure we have all the necessary #derives :) (I think this is done)

So I think we can clean this all up with another minor version.

[tcharding]

Just noting that the first step is where we got stuck last time, the naming of Even/Odd and not knowing how this maps to the integer value.

[dr-orlovsky]

Hm, isn't it quite obvious: odd should be a first odd integer value - and even should be a first even integer value, i.e. odd is 1 and even is 0.

0 is indeed even, since 0 % 2 == 0, and by definition even are the integers which divide by 2 without reminder.

[apoelstra]

I agree that the mapping between 0/1 and even/odd is natural .. I'm not sure how this maps to the 02/03 prefixes on full keys, but I should hope also in the natural way!

[elichai]

Yeah, in libsecp 0 is also used for even and 1 for odd.
(and in compressed 0x02 is for even and 0x03 is for odd)

I'm still somewhat uncomfortable exposing to users that public keys can even be "Even" or "Odd", but maybe this ship has long sailed.

[tcharding]

TIL: Cannot deprecate implementation of From.

#[deprecated(since = "0.21.3", note = "Please use Parity::from_i32")]
impl From<i32> for Parity {
    fn from(parity: i32) -> Parity {
        Parity::from_i32(parity)
    }
}

Results in:
error: this #[deprecated] annotation has no effect
--> src/key.rs:928:1
|
928 | #[deprecated(since = "0.21.3", note = "Please use Parity::from_i32")]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the unnecessary deprecation attribute
|
= note: #[deny(useless_deprecated)] on by default