Use fullyDefinedType for mirror synthesis

[bishabosha]

fullyDefinedType is used by most factories in Synthesizer, so why not try with mirrors :)

fixing the test case was tricky because recursive implicit search isn't so simple for a contravariant typeclass. But I included an example of breaking the recursive knot, while limiting the amount of code generation

fixes #13146

[bishabosha]

ok, investigation is required to see why this fails for higher kinded types.

Edit: stripTypeVars is necessary to instantiate type lambdas, so we keep it, but perform after fullyDefinedType

[bishabosha]

This isn't really a regression - it never worked in 3.0.0