Boot genesis block helper

Before this change, if you wanted to use a new Transaction
feature in the genesis block, you'd need to extend its
Creator object and associated methods.  With yesterday's
addtions to Transcation, it's now so easy to work with
Transactions directly that we can get rid of the middleman.

Also added a KeyPair type alias, so that ring could be easily swapped
out with a competing library, if needed.

src/signature.rs

@@ -6,18 +6,19 @@ use ring::signature::Ed25519KeyPair;
 use ring::{rand, signature};
 use untrusted;
 
+pub type KeyPair = Ed25519KeyPair;
 pub type PublicKey = GenericArray<u8, U32>;
 pub type Signature = GenericArray<u8, U64>;
 
 /// Return a new ED25519 keypair
-pub fn generate_keypair() -> Ed25519KeyPair {
+pub fn generate_keypair() -> KeyPair {
     let rng = rand::SystemRandom::new();
     let pkcs8_bytes = signature::Ed25519KeyPair::generate_pkcs8(&rng).unwrap();
     signature::Ed25519KeyPair::from_pkcs8(untrusted::Input::from(&pkcs8_bytes)).unwrap()
 }
 
 /// Return the public key for the given keypair
-pub fn get_pubkey(keypair: &Ed25519KeyPair) -> PublicKey {
+pub fn get_pubkey(keypair: &KeyPair) -> PublicKey {
     GenericArray::clone_from_slice(keypair.public_key_bytes())
 }