API Annotations : add Locations in hsSyn where layout occurs
At the moment ghc-exactprint, which uses the GHC API Annotations to provide a framework for roundtripping Haskell source code with optional AST edits, has to implement a horrible workaround to manage the points where layout needs to be captured.
These are
MatchGroup
HsDo
HsCmdDo
HsLet
LetStmt
HsCmdLet
GRHSs
To provide a more natural representation, the contents subject to layout rules need to be wrapped in a SrcSpan.