Use neseted initializer lists to construct matrix objects.

Add constructor of initializer_list<initializer_list<ex>> to matrix.
Use this syntax where, previously, ctor from comma-separated list of
elements was used. Deprecate the ctor from comma-separated list.

Note: The output format '[[a,b],[c,d]]' and ginsh syntax are
unchanged because lists are printed '{a,b,c}' and a matrix is not a
list of lists.