Improve performance of index-only scans with many index columns.

StoreIndexTuple was a loop over index_getattr, which is O(N^2)
if the index columns are variable-width, and the performance
impact is already quite visible at ten columns.  The obvious
move is to replace that with a call to index_deform_tuple ...
but that's *also* a loop over index_getattr.  Improve it to
be essentially a clone of heap_deform_tuple.

(There are a few other places that loop over all index columns
with index_getattr, and perhaps should be changed likewise,
but most of them don't seem performance-critical.  Anyway, the
rest would mostly only be interested in the index key columns,
which there aren't likely to be so many of.  Wide index tuples
are a new thing with INCLUDE.)

Konstantin Knizhnik

Discussion: https://postgr.es/m/e06b2d27-04fc-5c0e-bb8c-ecd72aa24959@postgrespro.ru