Implementation of cluster paper grammar and runtime

[KeyboardNerd]

This is a huge change to PILOTS runtime and compiler. I'll separate each step into small commits.
This is a breaking change and so merge the pull request with caution.

I've finished:

1. Restructuring existing runtime backend.
2. Trainer Lexer/Parser
3. Trainer CodeGen

I'll finish:

1. Estimator Runtime separation ( in separated PR )
2. Modification to existing PILOTS program compiler. ( in separated PR )
3. Fix broken Predictive Server ( in this PR )