For types the use of a string is ambiguous -- it could be either
an Identifier or a Name. Don't guess.
Retain the implicit promotion to Identifier in places where only
Identifier is legal, e.g. various symbol names.
With the upcoming addition of intersection types, a type can
be Identifier|Name|NullableType|UnionType|IntersectionType, which
is quite the mouthful. Give NullableType and UnionType a common
base class ComplexType, which does not have any behavior, but
allows to write these types (and check them in instanceof) more
easily.
Adds addAttribute() method to Builders of all nodes supporting attributes with BuilderHelpers::normalizeAttribute() usage inside so we can pass both Node\Attribute and Node\AttributeGroup instances.
