The dispatch using $this->{'reduceRule' . $rule}() is very expensive
because it involves
* One allocation when converting $rule to a string
* Another allocation when concatenating the two strings
* Lowercasing during the method call
* Various uncached method checks, e.g. visibility.
Using an array of closures for semantic action dispatch is
significantly more efficient.
We're accessing $this->stackPos a lot, and property accesses are
much more expensive than local variable acesses. Its cheaper to
use a local var and pass it as an argument to semantic actions.
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.
