Make generic and TH derivation for Unbox compatible

[harendra-kumar]

Generic takes 256 constructors max, TH can do more. We should use variable length encoding for constructor tags in TH and limit the generic constructors to 128 with encoding compatible to TH, and emit an error if that exceeds.

[adithyaov]

TH already does variable-length encoding.

Does Generic have variable length encoding? If so, they are compatible.

I need to understand the generic encoding before proceeding with how to change this.