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

Merging #55 (7bb1179) into master (2467caf) will not change coverage.
The diff coverage is 100.00%.

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

Impacted Files	Coverage Δ
queuelib/__init__.py	100.00% <100.00%> (ø)

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[Gallaecio]

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