Start on HasAndBelongsToMany

[shmax]

An attempt at implementing HasAndBelongsToMany (which was marked with "TODO: implement" in the original code, and evidently never completed) functionality:

• create new Student and Courses fixtures (every student attends many courses, every course is attended by many students)
• remove get_relationship stuff from RelationshipTest (a horrible anti-pattern, IMO)
• create HasManyAndBelongsToMany type, and convert to a array<string, HasManyAndBelongsToMany> format (for easier static checking)

Note that this is just the read part of this; I'm not sure what to do about writes, because as far as I can tell none of the other relationship classes currently support writes (ie build and create don't actually create new records).

[codecov]

Codecov Report

Merging #76 (f91b12a) into master (aeebc0d) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #76      +/-   ##
============================================
+ Coverage     96.26%   96.55%   +0.29%     
- Complexity      960      962       +2     
============================================
  Files            33       33              
  Lines          2328     2351      +23     
============================================
+ Hits           2241     2270      +29     
+ Misses           87       81       -6     

Files Changed	Coverage Δ
lib/Model.php	98.03% <ø> (ø)
lib/Relationship/AbstractRelationship.php	99.24% <100.00%> (ø)
lib/Relationship/HasAndBelongsToMany.php	86.20% <100.00%> (+86.20%)	⬆️
lib/Table.php	97.66% <100.00%> (+0.96%)	⬆️

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