Make importing memory queues from the package possible

[CrazyPython]

Right now to use the memory queues, we have to do this:

from queuelib.queue import FifoMemoryQueue, LifoMemoryQueue

This PR lets us do this:

from queuelib import FifoMemoryQueue, LifoMemoryQueue

This has readability gains because it means we can write:

from queuelib import FifoMemoryQueue, LifoMemoryQueue, PriorityQueue

Instead of:

from queuelib.queue import FifoMemoryQueue, LifoMemoryQueue, PriorityQueue
from queuelib.pqueue import PriorityQueue

Unless this is intentional to discourage using non-disk queues, this change should improve library ergonomics.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2467caf) to head (7bb1179).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #55   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          310       310           
  Branches        38        38           
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  Hits           310       310           

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[Gallaecio]

I wonder if we should take the chance to cover them in the README as well, for discoverability.