Add Display impl for ZendObject

[vodik]

Small PR that brings part of my work getting eval/compile support built.

Adds some basic support for capturing PHP exceptions, function calling, and convenience methods around converting from Zval to Rust types.

Which that groundwork in place, add a PHP equivalent implementation for Display.

[vodik]

Okay, so I think there's a lot of good stuff in here that should get merged in to get iterated on:

• Zval::extract as a QoL improvment
• ExecutorGlobals::take_exception
• FromZendObject/ZendObject::extract for implementing casts from objects.

That said, with all this built, I also think the Display implementation is inappropriate given that it can panic and I think should be dropped from this PR outright.

Thoughts?

[davidcole1340]

TryFrom<Zval> is not implemented for all types AFAIK, what about T: FromZval<'a> taking &'a self instead?

Or if you want to drop self then for<'a> T: FromZval<'a> and keep the function parameter as self. The for<'a> just makes it valid to have T as String and not &str (as it would reference the zval after dropping).