Change contract example to "playground"

[Alex Beregszaszi]

play.rust-lang.org is a very good example.

Our goal with this could be to showcase/highlight the different compiler features we have. We could do this before others adopt the given feature. Such features include:

• the IR and displaying the various stages of the code (Solidity, IR, optimised IR, assembly),
• the SMT checker,
• various Natspec features,
• AST
• etc.

(Not saying it should do all these at once, but as a potential feature set for the future.)

Likely I would consider this living on a different URL and use a small embedded version on the main page.