Make RecoveryId an enum

[liamaharon]

Closes #727

• Refactors RecoveryId into an enum.
• Replaces custom type methods from_i32 and to_i32 with TryFrom<i32> and Into<i32> (via From<RecoveryId> for i32) implementations.
• Removes derive Ord PartialOrd and Hash, they don't appear to be used. I can implement on the enum if we want to keep them.

[apoelstra]

In f6a76af:

There is an as i32 in your diff that can be converted to a i32::from. (The old code is at fault, but you're changing this line anyway so you should fix it.)

[liamaharon]

Good catch @apoelstra, fixed it

[liamaharon]

Force pushed a lint fix

[apoelstra]

ACK fa0c086 successfully ran local tests

[tcharding]

ACK fa0c086