Strip out isos

[vil1]

This PR changes the encoding once again, and simplifies things a bit. The main changes are:

• the IsoSchema case of SchemaF has been deleted
• SchemaZ now carries an iso from Repr (the concrete representation of the subject type in terms of "sum of products") to A (the subject type that is represented by this SchemaZ).

So now, a SchemaZ is composed of a "simple" structure (a Fix[SchemaF]) that represents the subject type in terms of "sum of products" and an isomorphism from this representation to the subject type itself.

[codecov-io]

Codecov Report

Merging #71 into prototyping will increase coverage by 0.09%. The diff coverage is 69.12%.

@@               Coverage Diff               @@
##           prototyping      #71      +/-   ##
===============================================
+ Coverage        70.54%   70.63%   +0.09%     
===============================================
  Files               18       18              
  Lines              516      521       +5     
  Branches             6        9       +3     
===============================================
+ Hits               364      368       +4     
- Misses             152      153       +1

Impacted Files	Coverage Δ
...ules/play-json/src/main/scala/PlayJsonModule.scala	0% <0%> (ø)	:arrow_up:
modules/core/src/main/scala/Json.scala	92.85% <100%> (ø)	:arrow_up:
modules/scalacheck/src/main/scala/GenModule.scala	90% <100%> (ø)	:arrow_up:
...dules/tests/src/main/scala/GenModuleExamples.scala	95.23% <100%> (ø)	:arrow_up:
...dules/tests/src/main/scala/MigrationExamples.scala	83.78% <100%> (ø)	:arrow_up:
...odules/tests/src/main/scala/GenericGenModule.scala	81.25% <100%> (+1.25%)	:arrow_up:
modules/generic/src/main/scala/ShowModule.scala	94.73% <100%> (-5.27%)	:arrow_down:
...es/tests/src/main/scala/SchemaModuleExamples.scala	100% <100%> (+9.09%)	:arrow_up:
...odules/generic/src/main/scala/GenericAlgebra.scala	91.66% <100%> (-0.65%)	:arrow_down:
modules/core/src/main/scala/migrations.scala	37.5% <39.13%> (+4.16%)	:arrow_up:
... and 5 more

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