Upgrade to Scala.js 1.0.0-RC1.

There were several changes to the IR in 1.0.0-RC1 that have an
impact on this codebase:

* Primitive types now have dedicated `PrimRef`s when used as
  `TypeRef`s, so for example `IntRef` instead of `ClassRef("I")`.
  See scala-js/scala-js#3802
* Names now have dedicated types instead of `String`s, and depend
  on the kind of symbol: `LocalName`, `LabelName`, `FieldName`,
  `MethodName` and `ClassName`. There are different kinds of
  `js.Ident`s to go with each kind of `Name`.
  See scala-js/scala-js#3804,
  scala-js/scala-js#3814 and
  scala-js/scala-js#3833
* Original names are now represented by a dedicated `OriginalName`
  type, and they are stored only at definition site, not at use
  site.
  See scala-js/scala-js#3845

Author: sjrd