Added @passthrough_property macro to resolve Issue #5 "EXTPassthrough breaks with implicit property synthesis"

[woolsweater]

The new macro allows forwarding of a specified property to a property on another class. This commit includes tests for the new macro, and a doc string.

This necessitated some small changes to the existing internal passthrough_ macro; in particular, a SEL_GETTER must be passed in — this is @selector() when the method name is specified literally, but sel_getUid() for properties, because the property’s accessor method names must be looked up at runtime. The lookup is implemented by two helper functions, which are in EXTRuntimeExtensions. I considered macro-generated functions to cache the accessor names, but it seemed more trouble than it was worth.

This commit is intended to resolve #5. It does so by inserting an @dynamic directive for the named property. This comes with the hitch that both the @property declaration and the @passthrough_property must be in the same "segment" of the class definition -- either the same category, or in the main @interface/@implementation blocks.

[jspahrsummers]

Thanks for submitting this! I'm interested to take a look, but libextobjc's experimental branch is honestly kinda low on my list of priorities.

I'll try to get around to it eventually. It's certainly awesome to see someone else hacking on it. 🌟

[woolsweater]

For sure! Somebody pointed this library out to me a month or two ago, and it seemed like exactly the kind of runtime hacking that I enjoy, so I just looked around for an issue to work on.

Hope it fits the bill whenever you get around to reviewing it, but I'm happy to make changes if necessary too.