WIP: concrete generics

[sobolevn]

Closes #24
Closes #255
Closes #260
Closes #259

[codecov-commenter]

Codecov Report

Merging #256 (20b7b58) into master (8842dff) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #256   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines           91        99    +8     
  Branches        11        15    +4     
=========================================
+ Hits            91        99    +8     

Impacted Files	Coverage Δ
classes/_registry.py	100.00% <100.00%> (ø)
classes/_typeclass.py	100.00% <100.00%> (ø)

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