InstantiateDependentMap could be lazier in calling `actuals.toIndexedSeq`

[retronym]

• Use and Array rather than Vector for actuals
• Just leave actuals and existentials null until first access (don't use a lazy val for this)
• Hopefully this doesn't happen most of the time as most types won't contain SingleType(NoPrefix, _) in the following match. Confirm that this is the case by observing what code paths get into StabilizedArgTp (with logging or a debugger).

def apply(tp: Type): Type = tp match {
      case SingleType(NoPrefix, StabilizedArgTp(tp)) => tp
      case _                                         => mapOver(tp)
    }