Simplify toExternalString for mutable types

The error message was essentially the same for mutable lists and
mutable hash tables, so we combine them.  We also drop
toExternalString(Type), since Type is a subclass of MutableHashTable,
and we can just use inheritance.

M2/Macaulay2/m2/nets.m2

@@ -69,14 +69,13 @@ toExternalString Net := x -> if height x + depth x == 0 then
      concatenate("(horizontalJoin())", "^", toString height x) else
      concatenate(format toString x, "^", toString(height x - 1))
 
-toExternalString MutableHashTable := s -> (
-     if hasAttribute(s,ReverseDictionary) then return toString getAttribute(s,ReverseDictionary);
-     error "anonymous mutable hash table cannot be converted to external string";
-     )
-toExternalString Type := s -> (
-     if hasAttribute(s,ReverseDictionary) then return toString getAttribute(s,ReverseDictionary);
-     error "anonymous type cannot be converted to external string";
-     )
+toExternalString MutableHashTable :=
+toExternalString MutableList      := s -> (
+    if hasAttribute(s,ReverseDictionary)
+    then toString getAttribute(s,ReverseDictionary)
+    else error("anonymous ", synonym class s,
+	" cannot be converted to external string"))
+
 toExternalString HashTable := s -> (
      concatenate (
 	  "new ", toExternalString class s,
@@ -86,10 +85,6 @@ toExternalString HashTable := s -> (
 	  then demark(", ", apply(pairs s, (k,v) -> toExternalString k | " => " | toExternalString v) )
 	  else "",
 	  "}"))
-toExternalString MutableList := s -> (
-     error "anonymous mutable list cannot be converted to external string";
-     -- concatenate("new ",toExternalString class s, " from {...", toString(#s), "...}" )
-     )
 mid := s -> (
      if #s === 1 then toExternalString s#0
      else between(",",apply(toSequence s,x -> if x === null then "" else toExternalString x))