Refactor index cost estimation functions in view of IndexClause changes.

Refactor index cost estimation functions in view of IndexClause changes.

Get rid of deconstruct_indexquals() in favor of just iterating over the
IndexClause list directly.  The extra services that that function used to
provide, such as hiding clause commutation and associating the right index
column with each clause, are no longer useful given the new data structure.
I'd originally thought that it'd provide a useful amount of abstraction
by freeing callers from paying attention to the exact clause type of each
indexqual, but that hope proves to have been vain, because few callers can
ignore the semantic differences between different clause types.  Indeed,
removing it results in a net code savings, and probably some cycles shaved
by not having to build an extra list-of-structs data structure.

Also, export a few formerly-static support functions, with the goal
of allowing extension AMs to write functionality equivalent to
genericcostestimate() without pointless code duplication.

Discussion: https://postgr.es/m/24586.1550106354@sss.pgh.pa.us