Store row and column as Word16 in SourcePos
Store row and column as Word16 in SourcePos

SourcePos now stores row and column as Word16 because that's the types
Parse.Primitives uses. Conversions still take place when the adapter API
is used (`sourceLine` and `sourceColumn` converts the Word16 to Int),
but conversion is not necessary internally.

avh4/elm-format,avh4/elm-format,avh4/elm-format,avh4/elm-format