Clean up Array concat() non-existent trailer test
Test for Array map() trailing non-existent elems

The real world behavior is to call the user callback only for elements
that exist, and to count any trailing non-existent elements into the
result array length.
Implement real world Array map() behavior
Array map() test case fixes and improvements

Fix plain map() test case, it broke because of new real world compatible
map() trailing non-existent elements behavior.  Add more cases to the
specific test case for trailing elements.
Add non-Array case for Array concat() trailer test

Since concat() doesn't "flatten" non-Array arguments, the result is simply
an array with two objects.
Guide update for array map() real world behavior
Release note: Array map() real world behavior
Makefile trivia: commented out option for map()
Merge branch 'nonstd-array-map'

Add real world behavior for Array.prototype.map(): trailing non-existent
elements don't invoke the user callback but are counted towards the
result array length.