Implemented is_map beam instruction in HiPE.
Added map rule to t_subtract/2 in lib/hipe/cerl/erl_types.erl

This prevents an infinite loop caused by the last case of t_subtract/2
assuming that one of it's arguments must be an union, which was broken
by the addition of a map type without adding a rule to t_subtract/2.

The rule is a simple overapproximation that returns the LHS as-is.