Retain comments on blocks on first inner statement

Add string kinds and doc string labels

Scalar\String_ and Scalar\Encapsed now have an additional "kind"
attribute, which may be one of:

* String_::KIND_SINGLE_QUOTED
* Str

Add string kinds and doc string labels

Scalar\String_ and Scalar\Encapsed now have an additional "kind"
attribute, which may be one of:

* String_::KIND_SINGLE_QUOTED
* String_::KIND_DOUBLE_QUOTED
* String_::KIND_NOWDOC
* String_::KIND_HEREDOC

Additionally, if the string kind is one of the latter two, an
attribute "docLabel" is provided, which contains the doc string
label (STR in <<<STR) that was originally used.

The pretty printer will try to take the original kind of the string,
as well as the used doc string label into account.

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Fix nop statement comment assignment

Keep around the start attributes on the lookahead token around in
a separate parser property.

Introduce Nop statement to collect dangling comments

A Nop statement will be inserted into statement lists if there are
any trailing comments in the list (which would otherwise not be

Introduce Nop statement to collect dangling comments

A Nop statement will be inserted into statement lists if there are
any trailing comments in the list (which would otherwise not be
associated with any node).

The pretty printer output currently still contains a superfluous
newline.

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Introduce Scalar\EncapsedStringPart

Add support for unicode escape sequences

Only parsed if the PHP 7 parser is used.

Move token constants into separate class

As these are shared between Php5 and Php7 parsers they should be
in some common place, otherwise we'd have to always reference either
one or

Move token constants into separate class

As these are shared between Php5 and Php7 parsers they should be
in some common place, otherwise we'd have to always reference either
one or the other.

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Implement generalized yield operator

And split tokens.y off, so I don't waste time debugging this again...

Test PHP 5 and PHP 7 parsers

At this point they should return the same result.

Fork separate PHP 7 parser

Also add ParserInterface