initialize from github.com/fufuok/utils/xsync

Expose map size method (puzpuzpuz#40)

Also updates Golang versions in GHA config.

Remove 1.16 from GHA config

Use staticcheck instead of golint (puzpuzpuz#35)

Cache top hash code MSBs in Map buckets (puzpuzpuz#31)

Includes the following:
* Remove linked lists from Map buckets
  - Also increases bucket size to fit 7 entries (2 CLs). So, when
    compared with the current Map maximum number of entries in
    bucket decrease from 9 to 7 entries. On the other hand, spacial
    locality and data density improved
* Add a cache with top bytes of entry hash codes
  - Improves key lookup efficiency by caching one most significant
    byte of each map entry. This way string dereference and equality
    check is avoided for a significant portion of scanned entries
* Decrease the shrink threshold fraction proportionally to the new
   map layout

Fix race in Map.LoadOrStore (puzpuzpuz#24)

Fix deadlock in Map.LoadOrStore (puzpuzpuz#23)

Also adds more tests for Map.

Fix MPMCQueue readme example (puzpuzpuz#15)