Add static_map::insert_or_apply aka reduce-by-key

[sleeepyjack]

This PR adds a function static_map::insert_or_apply, which either inserts a new key-value pair into the map, or, in case the key already exists, applies a reduction function over the associated value.

This effectively replaces the former cuco::static_reduction_map and thus superseeds #98.

Closes #82