Add multithreading-access tests

[chuwy]

Not yet sure what the test would be, but implementation doesn't look thread-safe right now.

[chuwy]

...or does it even matter since we don't have non-atomic operations, e.g. get-or-set?

[voropaevp]

This is most definitely not-thread-safe. An example in Readme is particularly prone to racing. As get and set could be called at different times, with the content of the map drifting.

Look at the MapRef from CE3 std and Chris Davenport's LruMap.