Centralize executor-related partitioning code.

Centralize executor-related partitioning code.

Some code is moved from partition.c, which has grown very quickly lately;
splitting the executor parts out might help to keep it from getting
totally out of control.  Other code is moved from execMain.c.  All is
moved to a new file execPartition.c.  get_partition_for_tuple now has
a new interface that more clearly separates executor concerns from
generic concerns.

Amit Langote.  A slight comment tweak by me.

Discussion: http://postgr.es/m/1f0985f8-3b61-8bc4-4350-baa6d804cb6d@lab.ntt.co.jp