Add bar position to case

[nojaf]

This is useful in recovery scenarios:

let x  = 
    switch y {
        | None => 0
        |
    }

[
  structure_item 
    Pstr_value Nonrec
    [
      <def>
        pattern 
          Ppat_var "x" 
        expression 
          Pexp_match
          expression 
            Pexp_ident "y" 
          [
            <case>
            | [3,25+8]
              pattern 
                Ppat_construct "None" 
                None
              expression 
                Pexp_constant PConst_int (0,None)
            <case>
            | [4,45+8]
              pattern 
                Ppat_extension "rescript.patternhole"
                []
              expression 
                Pexp_extension "rescript.exprhole"
                []
          ]
    ]
]

we can pinpoint a cursor position between the bar and the end of the switch.
Easily telling us that we are inside an empty case.

[zth]

Great stuff! @cristianoc any comments?

[cristianoc]

Looks great