Shorten variable prefix for P256K1 types from `P256k1` to two letters

[msgilligan]

P256k1 as used in P256k1PrivKey, P256k1PubKey, etc. is a little unwieldy. And it is already inconsistent -- some instances are P256k1 and others are P256K1 (with a capital K)

Java Crypto and Bouncy Castle both use EC for Elliptic Curve. We are using the P256k1 subset of EC, so some possible two-letter prefixes are:

1. PK -- P256K1
2. SP -- SecP
3. SJ -- Secp256k1-Jdk

I do think the prefix is needed because we already have collisions between the Java Crypto ECPoint and the Bouncy Castle ECPoint, etc. And our types will need to co-exist in the same source files.

I'm looking for feedback. Should we shorten to two-letters? If so, which two letters? I'm open to a three letter combination, too. Maybe SPK?

@schildbach @craigraw ?

[msgilligan]

I think I like SP the best.

[craigraw]

Of the options, I would probably do SJ - seems the most defendable choice.

[schildbach]

PK feels a bit misleading because I read "private key".

I think I'd vote for SP too. But I haven't yet thought about more alternatives.

[schildbach]

At the same time, I've got no problem with the full P256K1. At least it's very recognizable. In this case I'd just make casing consistent.