Modify array operations to include array's element type OID in the

Modify array operations to include array's element type OID in the
array header, and to compute sizing and alignment of array elements
the same way normal tuple access operations do --- viz, using the
tupmacs.h macros att_addlength and att_align.  This makes the world
safe for arrays of cstrings or intervals, and should make it much
easier to write array-type-polymorphic functions; as examples see
the cleanups of array_out and contrib/array_iterator.  By Joe Conway
and Tom Lane.