ExtRepOfObj for IsLetterAssocWordRep: return mutable list

... as this is how GAP behaved for years, and also many (most?)
other `ExtRepOfObj` implementations also return a mutable list.
So it seems plausible to mimic that.

At the same time, keep the return value of `ERepLettWord` immutable,
to avoid many needless copies of a list in code that does things
like `Length(ERepLettWord(w))` or `ERepLettWord(w)[2*n-1]`.

lib/wordlett.gi

@@ -171,7 +171,7 @@ local  i,r,elm,len,g,h,e;
 end);
 
 InstallMethod(ExtRepOfObj,"assoc word in letter rep",true,
-  [IsAssocWord and IsLetterAssocWordRep],0,ERepLettWord);
+  [IsAssocWord and IsLetterAssocWordRep],0,x->ShallowCopy(ERepLettWord(x)));
 
 InstallMethod(NumberSyllables,"assoc word in letter rep",true,
   [IsAssocWord and IsLetterAssocWordRep],0,

tst/testinstall/wordrep.tst

@@ -306,9 +306,9 @@ true
 #
 gap> f:= FreeGroup(IsLetterWordsFamily,4);;
 gap> IsMutable(ExtRepOfObj(f.1));
-false
+true
 gap> IsMutable(ExtRepOfObj(f.1));
-false
+true
 
 #
 gap> STOP_TEST("wordrep.tst");