Don't use reflection to handle enums

It turns out that changes I made earlier didn't work properly, and always caused a `CastClastException` to be thrown.

I've replaced the use of reflection by simply calling `At.valueOf` in `InjectionPoint` with the string that (the renamed method) `getAnnotationEnumName` returns.

I've tested this out with `Sponge` to ensure that it works properly.