Codepaths to remove in the future

[chenglou]

Here are some temporary codepaths we should remove one day:

• Our conversion of Js.t (#291, #317)
• Attribute and extension point bs.* conversion (#294)
• react jsx ppx 3 (#124, #159)
• if-let (#12) not that we'll remove it, just that it's pending, so we should be aware of it
• if we ever truly stop using some keywords:
  • to, downto (#327)
  • with (#329)
• Conversion of Pexp_apply("##, arg1, arg2) to Pexp_send (https://github.com/rescript-lang/syntax/pull/348) and https://github.com/rescript-lang/syntax/blob/1680ae5d0e5546f5a150e028a148aeee257bcd20/src/res_printer.ml#L3610