Add Annotations to the AST to simplify source to source conversions

Add Annotations to HsSyn to explicitly track the locations of all non-blank source code elements to allow tools to parse a Haskell file, modify the AST and then produce an updated version of the source preserving the layout for unchanged parts.

Discussion of the feature is at GhcAstAnnotations

Note: an early effort was at D246, but this was abandoned as unworkable.