Created by: helinwang

When looking at the ProgramDescBind, my first impression is it's only an auxiliary class for ProgramDesc. But actually it's the real "ProgramDesc" that gets passed between modules (e.g., gets passed into Executor), the current ProgramDesc is just a proto c++ class for the wire format.

I think to avoid confusion, we should rename ProgramDescBind (already lives in program_desc.cc) to ProgramDesc, and put the current ProgramDesc under proto namespace (or rename to ProgramDescProto).

How do you guys think?