Read a delivery receipt's body as the octets that arrived (#81)

* Regression tests for a receipt body read under a data_coding it is not written in

* Read a delivery receipt's body as the octets that arrived, not by its data_coding

* Assert SMPPSim's UCS2 receipt body parses like every other segment's

* Record the receipt-body defect as fixed

* Retain a multipart segment's octets once, not twice

* Sort the new type import into its file's order

src/udh.ts

@@ -9,6 +9,11 @@ export class ConcatReference {
 	}
 }
 
+/** A user data header is as long as its first octet says, that octet included. */
+export function udhLength(message: Buffer): number {
+	return (message[0] ?? 0) + 1;
+}
+
 export type ConcatInfo = {
 	part: number;
 	reference: number;