Code cleanups:
- use lens
- make builders fully abstract outside CodeGenMonad and TypeDecl
- replace builders with DSL
- abstract newtype generation etc.
- unify declareSumType and declareAlgebraicType
Yet unsupported features:
- attribute/element groups
- restrictions other than enumeration and pattern
- namespaces (altough it may be easy to add)
- this would better be added at Xeno level.
- that also means that the following are ignored:
- targetNamespace
- requiring qualification
- attributes with same name but wrong namespace are taken
- FromXML should be moved to
xeno - keys and uniques
- imports and includes