Support direct calls to initializers

[kyouko-taiga]

Example of program that should be supported:

type A: Deinitializable {
  public fun foo<T>(_ x: T) {}
  public memberwise init
}

fun bar<T>(_ x: T) {
  var x: A
  A.init(self: &x)
}

The first step is to redefine init as a contextual token so that we support the syntax A.init. Currently, one workaround is to enclose init in back-quotes.