Include deadline in SignScriptInput

`SignScriptInput()` now signs the concatenation of the (encoded) deadline and Input fields. The deadline is not included in the script body. This means that in order to verify the signature the `DefaultScript` must access the Deadline field of the `ScriptInput` directly. This is made possible by a new `op_deadline()` opcode. This field is then concatenated to the rest of the input via a new `op_concat()` opcode, and the result is passed to `pk.Verify()`.