crypto/secp256k1: Add godocs, remove indirection in privkeys (#2017)

* crypto/secp256k1: Add godocs, remove indirection in privkeys

The following was previously done for creating secp256k1 private keys:

First obtain privkey bytes. Then create a private key in the
underlying library, with scalar exponent equal to privKeyBytes.
(The method called was secp256k1.PrivKeyFromBytes,
https://github.com/btcsuite/btcd/blob/fb90c334dffc6c91843d8d36ac31e7f7eb6c7594/btcec/privkey.go#L21)

Then the private key was serialized using the underlying library, which just
returns back the bytes that comprised the scalar exponent, but padded to be
exactly 32 bytes.
https://github.com/btcsuite/btcd/blob/fb90c334dffc6c91843d8d36ac31e7f7eb6c7594/btcec/privkey.go#L70

Thus the entire indirection of calling the underlying library can be avoided
by just ensuring that we pass in a 32 byte value. A test case has even be written
to show this more clearly in review.

* crypto/secp256k1: Address PR comments

Squash this commit

* crypto: Remove note about re-registering amino paths when unnecessary.

This commit should be squashed.

crypto/encoding/amino/amino.go

@@ -13,8 +13,9 @@ func init() {
 	// NOTE: It's important that there be no conflicts here,
 	// as that would change the canonical representations,
 	// and therefore change the address.
-	// TODO: Add feature to go-amino to ensure that there
-	// are no conflicts.
+	// TODO: Remove above note when
+	// https://github.com/tendermint/go-amino/issues/9
+	// is resolved
 	RegisterAmino(cdc)
 }