* [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

Rationale: printer is a common infra that is shared across all nodes.

* [NODE][REFACTOR] Rename IRFunctor->NodeFunctor, use function pointer for dispatching.

Previously we used std::function for the functor dispatching.
It introduces additional overhead and problems during dll destruction(of std::function).

This PR changes the std::function to function pointers.
This change a bit restrictions around the set_dispatch that we can get around,
but will improve the general efficiency by reducing one level of indirection in the std::function.
We also no longer need special marcos to register functions to the Functor.