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.
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.
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.
