pgsql: Add and use BitmapHeapScanDescData struct

Add and use BitmapHeapScanDescData struct

Move the several members of HeapScanDescData which are specific to
Bitmap Heap Scans into a new struct, BitmapHeapScanDescData, which
inherits from HeapScanDescData.

This reduces the size of the HeapScanDescData for other types of scans
and will allow us to add additional bitmap heap scan-specific members in
the future without fear of bloating the HeapScanDescData.

Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/c736f6aa-8b35-4e20-9621-62c7c82e2168%40vondra.me

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7a8fc10ccb882249f8624b937f2c3b467d07bd6

Modified Files
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src/backend/access/heap/heapam.c         | 48 ++++++++++++++++++++++----------
src/backend/access/heap/heapam_handler.c | 18 +++++++-----
src/include/access/heapam.h              | 24 ++++++++++------
src/tools/pgindent/typedefs.list         |  1 +
4 files changed, 61 insertions(+), 30 deletions(-)