Some work towards more map type information in HiPE and Dialyzer

 * Added t_is_singleton/2 and t_map_entries/2 help functions to
   lib/hipe/cerl/erl_types.erl
 * Added a case that would cause overflow in erl_types, throwing an
   informative message to whomever might need it.
 * Added find_terminals/1 case for maps in
   lib/dialyzer/src/dialyzer_dataflow.erl
 * Added a reference output to the dialyzer small_SUITE/maps1 case under
   the assumption that it's absence did not imply that dialyzer should
   find no discrepancies.
 * Added maps:get/2 to lib/hipe/cerl/erl_bif_types.erl, type/5.
Added pp_typed_icode output of maps_SUITE for use in development