[RFC] TVMScript Metaprogramming (#79)

* Add TVMScript Metaprogramming RFC

* Add clarification for host program interleaving

* Fix typo and formating

* Add clarification around parse-time eval

* Further clarify parse-time eval

* Add clarification on how F3 and F4 will be implemented by parse-time eval

* Clarify the signature of user-provided function

* Clarify allowed types for auto capture

* Add section for security implication of eval

* Add short discussion on Taichi and Triton

* Clarify on how the new approach improves tooling

* Clarify and fix typos