Messages: Introduce a ChosenChannel IE, the first TV

Make GSM0808IE>>#length: return the length of the payload
including the length field if it is present. This is done
to be able to support a Tag only IE in the future.

BSSMAP.st

@@ -3,7 +3,7 @@ IEBase subclass: GSM0808IE [
     <comment: 'Base class of IEs for GSM0808'>
 
     GSM0808IE class >> length: aByteArray [
-        ^ (aByteArray at: 2).
+        ^ (aByteArray at: 2) + 1.
     ]
 ]
 
@@ -241,3 +241,29 @@ GSM0808IE subclass: GSMCauseIE [
         aMsg putByte: cause.
     ]
 ]
+
+GSM0808IE subclass: GSM0808ChosenChannel [
+    | channel |
+
+    GSM0808ChosenChannel class >> elementId [ ^ 33 ]
+    GSM0808ChosenChannel class >> initWith: aChannel [
+        ^ self new
+            channel: aChannel;
+            yourself
+    ]
+
+    GSM0808ChosenChannel class >> length: aByteArray [
+        ^ 1
+    ]
+
+    GSM0808ChosenChannel class >> parseFrom: aByteArray [
+        ^ self initWith: (aByteArray at: 2).
+    ]
+
+    channel [ ^ channel ]
+    channel: aChannel [ channel := aChannel ]
+
+    writeOnDirect: aMsg [
+        aMsg putByte: channel
+    ]
+]

Messages.st

@@ -41,9 +41,9 @@ Object subclass: IEMessage [
                     ifTrue: [
                         | enc size |
                         size := each length: data.
-                        enc := data copyFrom: 1 to: 2 + size.
+                        enc := data copyFrom: 1 to: 1 + size.
                         aMsg addIe: (each parseFrom: enc).
-                        ^ 2 + size
+                        ^ 1 + size
                 ].
             ].