Add ErrorHandler interface, as well as ErrorHandler\Throwing
and ErrorHandler\Collecting. The error handler is passed to
Parser::parse(). This supersedes the throwOnError option.
NameResolver now accepts an ErrorHandler in the ctor.
These were necessary back in the day when we had to emulate some
complex functionality such as nowdoc strings. Now we can simply
directly translate certain token sequences.
The motivation for this change is to avoid preprocessing of the source
code, which would complicate offset-aware error handling inside the
lexer as offsets would no longer be correct.
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.
