Parser: Introduce StatementList + NewStatementList()

Adds new Traits::Type::StatementList definitions both for Parser and
PreParser, and the corresponding NewStatementList() factory function.
This is needed to be able to define in ParserBase parsing functions
that return and manipulate lists of statements.

Moving and renaming PreParser::Statement to PreParserStatement is also
needed so its definition is available earlier for PreParserStatementList
to use it.

R=marja@chromium.org

Review URL: https://codereview.chromium.org/252423007

Patch from Adrian Perez de Castro <aperez@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20965 ce2b1a6d-e550-0410-aec6-3dcde31c8c00