Add abc::map::begin() and end() ‒ will need const versions
Fix comment
Allow for inlining of abc::map methods involved in lookup
Prepare for dynamic neighborhood size in abc::map

The original paper on hopscotch hashing neglected discussing what happens
with a very subpar hash function resulting in more collisions than the
neighborhood size. The only fix that doesn’t involve structural changes is
to make the neighborhood size variable.