Add shortcuts to create concerns and handlers without boilerplate

[Kaliumhexacyanoferrat]

As a developer of a web application, I would like to add custom concerns and handlers without the need of implementing several interfaces.

Example

var concern = Concern.From((IRequest request, IHandler content) => request.Respond() ... );

var handler = Handler.From((IRequest request) => request.Respond() ... );

Acceptance criteria

• The functionality is provided by an already existing module, e.g. I/O or even API
• There are methods for async and sync handlers
• The feature is documented on the GenHTTP website
• The feature is covered by acceptance tests