Switch to even-y tiebreaker for pubkeys

[sipa]

Addresses #191.

TODO: adjust reference code, but we'll probably only want to do that after addressing #190 as well.

[real-or-random]

Concept ACK, looks good except nits. :)

• Can you define &? We define a lot of notation in this bip, and then for consistency it makes sense to define that one too.
• Should we call this evenness instead of oddness?

[ajtowns]

Can you define &?

Saying ** The function ''is_even(P)'', where ''P'' is a point, returns ''y(P) = 0 mod 2''. should work, we've already got blah ≠ 0 mod p elsewhere.

Should we call this evenness instead of oddness?

We only use "oddness" once when talking about choosing which R point, so doesn't seem worth worrying about to me.

[sipa]

Changed things a bit.

[real-or-random]

ACK :)

[ajtowns]

ACK

[sipa]

Fixed a few more remaining mentions of "point".