Reimplement array concatenation for Ada and D

Reimplement array concatenation for Ada and D

This started as a patch to implement string concatenation for Ada.
However, while working on this, I looked at how this code could
possibly be called.  It turns out there are only two users of
concat_operation: Ada and D.  So, in addition to implementing this for
Ada, this patch rewrites value_concat, removing the odd "concatenate
or repeat" semantics, which were completely unused.  As Ada and D both
seem to represent strings using TYPE_CODE_ARRAY, this removes the
TYPE_CODE_STRING code from there as well.