Implement map.equal/2

The unification operation on 2-3-4 trees tests if they are structurally
equal.  They might also be considered equal if their sets of key-value pairs
are equivalent.  I've created an equal/2 predicate in the tree234 and
map modules in the standard library to test this.

library/tree234.m:
    Implement a predicate to test for tree234 equivalence.

library/map.m:
    Forward map.equal/2 to tree234.equal/2.

NEWS:
    Announce this change in the news file.

tests/general/map_equal.m:
tests/general/map_equal.exp:
    Add test case for map.equal/2

tests/general/Mmakefile:
    Include the new test case.