First class handlers from handing functions

[ppolesiuk]

It would be nice to allow defining first class handlers by providing handling function. We agreed (more-less) on the following syntax:

handlerfn
  <def₁> ... <defₙ>
  in
  <expr_cap>
  return <pat_r> => <expr_r>
  finally <pat_f> => <expr_f>
end

The return and finally clauses are optional. The meaning of such a code is a first class handler that realizes the following handling function:

fn run =>
  let <pat_f> =
    <def₁> ... <defₙ>
    let <pat_r> = run <expr_cap>
    in
    <expr_r>
  in
  <expr_f>