Add DMUL and DDIV overload

[lupino3]

As a first step, simplify instruction building by using a dictionary rather than a giant switch statement.

Will fix #867.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.13084% with 2 lines in your changes missing coverage. Please review.

Project coverage is 52.04%. Comparing base (f110e35) to head (abf2c2d).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #871      +/-   ##
============================================
- Coverage     52.18%   52.04%   -0.15%     
+ Complexity     1459     1394      -65     
============================================
  Files           251      251              
  Lines         10248    10147     -101     
  Branches       1104     1104              
============================================
- Hits           5348     5281      -67     
+ Misses         4580     4546      -34     
  Partials        320      320              

Files	Coverage Δ
...java/org/edumips64/core/is/InstructionBuilder.java	98.37% <98.13%> (+14.44%)	⬆️

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