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.
In order to get rid of the flag in `BuilderHelpers::normalizeNameCommon()` I have moved all the logic related to the normalization of the name to the `BuilderHelpers::normalizeName()` method and expr-related stuff to the `BuilderHelpers::normalizeNameOrExpr()` method which later calls the basic `normalizeName()` as well
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.
The parser will now always generate Identifier nodes (for
non-namespaced identifiers). This obsoletes the useIdentifierNodes
parser option.
Node constructors still accepts strings and will implicitly create
an Identifier wrapper. Identifier implement __toString(), so that
outside of strict-mode many things continue to work without changes.
