Changed fastmap's $has() method to use C++ contains() method (which is new in hopscotch-map 2.3.0). (#30)

Previously calling $mset() with empty input would result in an error; now it is a no-op. (#38)

Updated hopscotch-map library to 2.3.0.

Closed #24: Added a $clone() method to fastmap. (#26)

Fixed #27: If a fastmap object has no holes in the lists storing keys and values, and then it is serialized and then unserialized, the new fastmap would contain zero items. (#28)

Faster implementations of fastmap $keys() and $as_list() methods.