Add test for mel.as in mel.set external

[jchavarri]

There used to be a way to rename polyvars in externals by using bs.set in combination with bs.as, as documented in melange.re#using-polymorphic-variants-to-bind-to-enums, e.g:

type document
type style

external document : document = "document" [@@bs.val]
external get_by_id : document -> string -> Dom.element = "getElementById"
  [@@bs.send]
external style : Dom.element -> style = "style" [@@bs.get]
external transition_timing_function :
  style ->
  [ `ease
  | `easeIn [@bs.as "ease-in"]
  | `easeOut [@bs.as "ease-out"]
  | `easeInOut [@bs.as "ease-in-out"]
  | `linear
  ] ->
  unit = "transitionTimingFunction"
  [@@bs.set]

let element_style = style (get_by_id document "my-id")
let () = transition_timing_function element_style `easeIn

I can't make this code work with the latest version, see the test included in the PR. In any combination of attributes (bs.sel, mel.set, bs.as, as, mel.as) the resulting JavaScript code does not honor the as attribute to rename the polyvar.

[jchavarri]

@anmonteiro pointed out I as missing mel.string, updated the tests to reflect the 2 situations (with and without mel.string).

[jchavarri]

See related: #578

[anmonteiro]

This is a good test case to have when we get around to implementing #578. Thanks!