Eval minor refactoring

[anton-trunov]

• Added monadic projections fstM and sndM
• In MonadUtils

  let%bind foo = bar in
  pure foo

  refactored into

  let%map foo = bar in foo

  This is especially useful when there are several independent let%bind expressions in a row because let%map also reflects that.
• Minor refactoring in Eval, this is mostly about using Env.bind_all because it's much easier to read compared to List.fold_left ....