Improve GIN cost estimation

GIN index scans were not taking any descent CPU-based cost into account.  That
made them look cheaper than other types of indexes when they shouldn't be.

We use the same heuristic as for btree indexes, but multiply it by the number
of searched entries.

Additionally, the CPU cost for the tree was based largely on a
genericcostestimate.  For a GIN index, we should not charge index quals per
tuple, but per entry. On top of this, charge cpu_index_tuple_cost per actual
tuple.

This should fix the cases where a GIN index is preferred over a btree and
the ones where a memoize node is not added on top of the GIN index scan
because it seemed too cheap.

We don't packpatch this to evade unexpected plan changes in stable versions.

Discussion: https://postgr.es/m/CABs3KGQnOkyQ42-zKQqiE7M0Ks9oWDSee%3D%2BJx3-TGq%3D68xqWYw%40mail.gmail.com
Discussion: https://postgr.es/m/3188617.44csPzL39Z%40aivenronan
Author: Ronan Dunklau
Reported-By: Hung Nguyen
Reviewed-by: Tom Lane, Alexander Korotkov

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