Replace transaction traits with structs

Also:
* SystemTransaction::new -> new_account
* SystemTransaction::new_create -> new_program_account

sdk/src/loader_transaction.rs

@@ -6,33 +6,27 @@ use crate::pubkey::Pubkey;
 use crate::signature::Keypair;
 use crate::transaction::Transaction;
 
-pub trait LoaderTransaction {
-    fn loader_write(
+pub struct LoaderTransaction {}
+
+impl LoaderTransaction {
+    pub fn new_write(
         from_keypair: &Keypair,
         loader: Pubkey,
         offset: u32,
         bytes: Vec<u8>,
         last_id: Hash,
         fee: u64,
-    ) -> Self;
-
-    fn loader_finalize(from_keypair: &Keypair, loader: Pubkey, last_id: Hash, fee: u64) -> Self;
-}
-
-impl LoaderTransaction for Transaction {
-    fn loader_write(
-        from_keypair: &Keypair,
-        loader: Pubkey,
-        offset: u32,
-        bytes: Vec<u8>,
-        last_id: Hash,
-        fee: u64,
-    ) -> Self {
+    ) -> Transaction {
         let instruction = LoaderInstruction::Write { offset, bytes };
         Transaction::new(from_keypair, &[], loader, &instruction, last_id, fee)
     }
 
-    fn loader_finalize(from_keypair: &Keypair, loader: Pubkey, last_id: Hash, fee: u64) -> Self {
+    pub fn new_finalize(
+        from_keypair: &Keypair,
+        loader: Pubkey,
+        last_id: Hash,
+        fee: u64,
+    ) -> Transaction {
         let instruction = LoaderInstruction::Finalize;
         Transaction::new(from_keypair, &[], loader, &instruction, last_id, fee)
     }