Multiplicity annotation on records

Needing to store multiplicity annotations on records triggered a refactoring of AST of data declarations:

Moved HsBangTy and HsRecTy from HsType to HsTypeGhcPsExt, the extension of HsType during parsing, since they are only needed during parsing.

New HsConDeclField that stores all source data shared by all constructor declaration fields: unpackedness, strictness, multiplicity, documentation and the type of the field.

Merged HsMultAnn and HsArrowOf, so all multiplicity annotations share the same data type.

HsBang was no longer needed as a separate type, and was inlined into HsSrcBang.