The new dereferencing syntaxes (new Foo)->bar and (new Foo)['bar'] were
causing a shift/reduce conflict with the '(' expr ')' rule. When
(new Foo) was encountered (without dereference operators following) the
parser thus threw a parse error.
The fix simply adds a special '(' new_expr ')' rule to expr. This does not
remove the shift/reduce conflict itself, but makes it irrelevant.
This fixes issue #20.
