[Net\serve] add support for Function param / request handler

[drkameleon]

Right now, serve works as some sort of opinionated Sinatra, written internally in Arturo.
This isn't that bad, but it's probably too restricting, being an stdlib function.

What we could do is:

• add support for Function params
• when the user passes a $[req][...] parameter, this function will receive a dictionary with all the request details
• and it will expect a request answer (= dictionary) as a response

This way we will allow for even more fine-tuned handling of the whole server.

Also, this would allow whatever Sinatra- or non-Sinatra-like alternative to be developed in the form e.g. of external packages.

As usual: ease of use, yes; but also, flexibility.
(Arturo should not be about showing us the ONE way to do things, but instead enabling us different paths to... the solution 😉 )