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.
Were this library to be fully annotated with scalar types and
return types where possible and were strict types to be enabled
for all files, the test suite would now pass.
