Use static callback for TypeTraverser::map()

[staabm]

No description provided.

[ondrejmirtes]

What's the benefit here?

[staabm]

not measurable as I can see it.

I came to this change because in php-src they seem to try hard to auto-detect "static callback" as it seems there can be some benefit in it: php/php-src#19941

I am fine with closing in case you don't like it

[ondrejmirtes]

It's okay :)

[iluuu1994]

Side-note: The caching of static closures has an additional restriction.

The PR also adds caching for static closures that don't have any bindings and don't declare static variables.

The binding disqualifies the closure from being cached, so this won't benefit either way.

[staabm]

Thanks for noting!