Refactor code for partition bound searching

Remove partition_bound_cmp() and partition_bound_bsearch(), whose
void * argument could be, depending on the situation, of any of
three different types: PartitionBoundSpec *, PartitionRangeBound *,
Datum *.

Instead, introduce separate bound-searching functions for each
situation: partition_list_bsearch, partition_range_bsearch,
partition_range_datum_bsearch, and partition_hash_bsearch.  This
requires duplicating the code for binary search, but it makes the
code much more type safe, involves fewer branches at runtime, and
at least in my opinion, is much easier to understand.

Along the way, add an option to partition_range_datum_bsearch
allowing the number of keys to be specified, so that we can search
for partitions based on a prefix of the full list of partition
keys.  This is important for pending work to improve partition
pruning.

Amit Langote, per a suggestion from me.

Discussion: http://postgr.es/m/CA+TgmoaVLDLc8=YESRwD32gPhodU_ELmXyKs77gveiYp+JE4vQ@mail.gmail.com