Krop Server-side Form Handling

[noelwelsh]

This issue collects work for server side form handling in Krop. This covers at least:

• extracting parameters from requests (in routes and controllers)
• creating forms (in views)

We discuss these in order below.

Extracting Form Parameters from Requests

This will build on http4s form handling, which starts with the UrlForm type. We already have codecs for http4s' UrlForm as an Entity. However this is the lowest level of structure a user would work with. I expect that most would prefer to deserialize a form to a named tuple or a case class.

Creating Forms

This is tied into #14.