Make the api fully polymorphic in the java side

[jneira]

• The haskell implementation of Normalize if polymorphic on both type parameters s and a
• Unfortunately the java wrapper cant "thread" the polymorphism from java to haskell using the foreign exports, So we have to write a method for each possible value of s and a and cant use the Normalizer or the TypeChecker (see Implement JavaConverter for a rank 2 type typelead/eta#886)
• We should:
  • Investigate what changes we should do in eta to allow that.
  • Separate the instance of JavaConverter and DeepJavaConverter for Expr s a: https://github.com/eta-lang/dhall-eta/blob/master/src/main/eta/Dhall/Eta/Core/Java.hs#L986