Improve developer ergonomics with easier access to query parameters, cookies, etc

[ajusa]

I recently ported over a small project of mine (https://github.com/ajusa/250) from Jester to Mummy, and ended up creating a file (https://github.com/ajusa/250/blob/master/src/mummy_utils.nim) which has several utility functions that I thought were "missing" from mummy itself.

It'd be nice if some of these could be added to mummy itself - or if there were easier ways to set cookies, do redirects, and parse objects from query parameters/post bodies. Those were my main pain points.

I believe you mentioned on Discord that you were exploring a pattern to handle parsing query parameters and path parameters, if that's the case that would probably fix most of the issues I've described above.