Nab some low-hanging fruit: replace the planner's base_rel_list and

Nab some low-hanging fruit: replace the planner's base_rel_list and
other_rel_list with a single array indexed by rangetable index.
This reduces find_base_rel from O(N) to O(1) without any real penalty.
While find_base_rel isn't one of the major bottlenecks in any profile
I've seen so far, it was starting to creep up on the radar screen
for complex queries --- so might as well fix it.