Add toExternalString(MutableMatrix)

We just use the same method as we use for mutable lists and hash
tables.

M2/Macaulay2/m2/mutablemat.m2

@@ -16,6 +16,7 @@ precision MutableMatrix := precision @@ ring
 expression MutableMatrix := m -> MatrixExpression append(applyTable(entries m, expression), symbol MutableMatrix => true)
 texMath MutableMatrix := m -> texMath expression m
 net MutableMatrix := m -> net expression m
+toExternalString MutableMatrix := lookup(toExternalString, MutableHashTable)
 
 map(Ring,RawMutableMatrix) := opts -> (R,m) -> (
      new MutableMatrix from {