Allow to specify #columns in heap/index_form_tuple

Hi,

The covering indexes patch [1] really needs a version of
heap_form_tuple/index_form_tuple that allows to specify the number of
columns in the to-be generated tuple.  Previously the faster expression
evaluation stuff could also have benefited form the same for both
forming and deforming tuples.

It's obviously trivial to add, codewise.

I think for index_form_tuple we can just add a parameter, but for
heap_form_tuple/heap_deform_tuple that'd probably break a number of
external users.  How about adding _ex variants that allow to specify
that?

Regards,

Andres

[1] http://archives.postgresql.org/message-id/56168952.4010101%40postgrespro.ru