Use OP_0 to better mimic bitcoin core code

Our `Builder::push_int` method is the same as Bitcoin Core `CScript`
`push_int64` method. We currently use `OP_FALSE` (equivalent to `OP_0`)
but recently we added `OP_0`, lets use it to make our code better mimic
Core (also saves devs checking that `OP_FALSE` is the same as `OP_0`).

bitcoin/src/blockdata/script/builder.rs

@@ -42,7 +42,7 @@ impl Builder {
         }
         // We can also special-case zero
         else if data == 0 {
-            self.push_opcode(opcodes::OP_FALSE)
+            self.push_opcode(opcodes::OP_0)
         }
         // Otherwise encode it as data
         else { self.push_int_non_minimal(data) }