Divide: implementation + dectest

[barucden]

Fixes #56

We now have:

julia> Decimal(10)^30 / Decimal(2)^100
0.7888609052210118054117285655

In Python:

>>> Decimal(10)**30 / Decimal(2)**100
Decimal('0.7888609052210118054117285655')

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (5cd504f) to head (2f1ec58).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   99.66%   99.68%   +0.01%     
==========================================
  Files          11       11              
  Lines         303      319      +16     
==========================================
+ Hits          302      318      +16     
  Misses          1        1              

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