Reuse Span defined in pretty

[harpocrates]

The project already has a dependency on pretty for pretty-printing. In Text.PrettyPrint.Annotated.HughesPJ, there is

data Span a = Span
                  { spanStart      :: !Int
                   , spanLength     :: !Int
                   , spanAnnotation :: a
                   } deriving (Show,Eq)

Which might be a good candidate as the output tag type for parsing. Some thoughts on this:

• One uniform Span data type for both pretty-printing and parsing is slick, especially a reused one.

• I'm not sure I see any use for spanAnnotation with parsing - maybe a means of plugging in more helpful information about position. For the latter, one would probably define

  data Pos = Pos { row :: !Int, col :: !Int } deriving (Eq, Ord)
  type ParseSpan = Span Pos
  

  then parsing functions would produce things of type Expr ParseSpan, Ty ParseSpan, etc.

All changes would be in the Parser module, just returning this type.

[harpocrates]

Given that as of #5 we are now using wl-pprint-annotated which doesn't have Span (it doesn't bother to define such a type), this is no longer on the table.