A simple struct containing a full position, including a partition key
and a position in partition. Two variants are introduced: an owning
version and a view. This is to replace all the ad-hoc structures
introduced for the same purpose: std::pair() and std::tuple() of
partition key and clustering key, and other similar small structs
scattered around the code.
This patch does not replace any of the above mentioned construcs with
the new full_position, it merely introduces it to enable incremental
standardization.
