* [REFACTOR] Establish printer in the source folder.

As we move towards the unified IR, we will eventually want to build a unified
printers for both relay and TIR.

This PR isolate the printer component into a separate folder in src as a first step.

- Refactored the Doc DSL using Object, clean up APIs.
- Isolate out the meta data into a header.
- move printer into relay_text_printer, add comments about further TODos.

* Rename NodePrinter -> ReprPrinter to distinguish it from other printers