added structs for statistics in backend
made frontend structs compatible
fix segmentation fault for null values
support for various integer types
support for char
find '=constant' selections while handling the null-tuple of sequential scan; next step: try it out for init prog node
moved implementation to execInitNode
Merge remote-tracking branch 'origin/selection_handler' into basestats
Merge remote-tracking branch 'origin/statistics_structs' into basestats

Conflicts:
	src/backend/executor/execProcnode.c
	src/backend/piggyback/piggyback.c