Add functional `solve` API, use it for tests

[tlambert03]

This includes #21 and would add a new functional ilpy.solve convenience function that, when used with expressions, can avoid a fair amount of boilerplate:

In [1]: import ilpy

In [2]: x = ilpy.Variable('x', index=0)

In [3]: ilpy.solve(x**2, constraints=[x>=3], sense='minimize', variable_type='integer')
Out[3]: [3.0]

it is also used in this PR for testing... adding a new Case tuple (like test case) that makes it easy to setup up testing a number of different programs

[codecov-commenter]

Codecov Report

Merging #22 (1c19b14) into main (9a74037) will decrease coverage by 1.33%. The diff coverage is 75.67%.

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@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   86.10%   84.78%   -1.33%     
==========================================
  Files           3        4       +1     
  Lines         331      368      +37     
==========================================
+ Hits          285      312      +27     
- Misses         46       56      +10     

Impacted Files	Coverage Δ
ilpy/_functional.py	74.28% <74.28%> (ø)
ilpy/__init__.py	84.61% <100.00%> (+2.79%)	:arrow_up:

... and 1 file with indirect coverage changes

[funkey]

Beautiful, I like it! :)