[cs] use PHP 5.4 short array, since PHP 7.0 is min version

[cs] remove unused imports

Generate PHP 7 type annotations

Always use Identifier nodes

The parser will now always generate Identifier nodes (for
non-namespaced identifiers). This obsoletes the useIdentifierNodes
parser option.

Node

Always use Identifier nodes

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.

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Change BuilderAbstract into BuilderHelpers static class

Used as poor man's function namespace here.