Include trailing semicolon in GroupUse

Fixes https://github.com/nikic/PHP-Parser/issues/1026.

Represent names using string rather than array of parts

In most circumstances we are interested in the whole string, not
the parts split by namespace separator. As names are common, this

Represent names using string rather than array of parts

In most circumstances we are interested in the whole string, not
the parts split by namespace separator. As names are common, this
representation measurably improves memory usage and performance.

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Rename Stmt\UseUse to UseItem

Distinguish between implicit/explicit alias

The UseUse::$alias node can now be null if an alias is not
explicitly given. As such "use Foo\Bar" and "use Foo\Bar as Bar"
are now repres

Distinguish between implicit/explicit alias

The UseUse::$alias node can now be null if an alias is not
explicitly given. As such "use Foo\Bar" and "use Foo\Bar as Bar"
are now represented differently.

The UseUse->getAlias() method replicates the previous semantics,
by returning "Bar" in both cases.

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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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Support !!positions parser test mode

And use it for the group use prefix position test that was
previously implemented as a separate test.