Reduce or remove custom class extension

[ckarenz]

We currently support class extension through a custom attribute ExtendClassAttribute, which Llilum handles in ApplyClassExtension. This mechanism exists to allow Llilum-specific runtime code to exist in a separate assembly from mscorlib, without polluting the existing CLR interfaces.

Unfortunately, this approach is a bit fragile and adds significantly to both complexity and build time. If possible, we would like to replace this with a more CLI-friendly way of doing things.