Separate form_urlencoded into its own crate

[jplatte]

Hi! I'm trying to trim Ruma's dependencies, and noticed that the url crate, of which we only use the form_urlencoded module (which AFAICT doesn't require any external dependencies) is the only reason for the inclusion of idna, unicode-bidi, unicode-normalization and smallvec in our dep graph. Would a PR separating it out into its own crate like percent-encoding be considered?

[valenting]

Splitting it out would probably be OK, and I think we can do it without breaking the API.

[jplatte]

Awesome! Will work on a PR as soon as I find the time.