Feature: Support assertions in RCL

[metasansana]

These could be preconditions that must be satisfied for a route to be executed properly. Failure would result in some error machinery being triggered.

Example:

GET /posts !allowed

Where "allowed" will be some middleware that simply preforms a check:

 const allowed = (req: Request) => req.session.has('allowed') ? pure(true) : pure(false)

Failing the assertion should hook into an on.assertion event that by default sends a 409 or 403 response.