varags broken with polymorphic types

[johnnovak]

Currently, it is not possible to pass in subtyped objects in varags arguments.

type
  Base = ref object of RootObj
  A = ref object of Base
  B = ref object of Base

proc test(oa: varargs[Base]) = discard

#test(A()))          # FAILS TO COMPILE:
                     # Error: type mismatch: got (A)
                     # but expected one of:
                     # proc test(oa: varargs[Base])

#test(A(), B())      # FAILS TO COMPILE:
                     # Error: type mismatch: got (A, B)
                     # but expected one of:
                     # proc test(oa: varargs[Base])