now-lambda: Better interface (use http/af request and response)

[anmonteiro]

This makes the now-lambda handler interface be the following:

type body = string

val handler
  :  Httpaf.Request.t * body option
  -> Lambda_runtime.Context.t
  -> (Httpaf.Response.t * body, string) result

Since in now.sh we're effectively handling HTTP requests, it makes sense to make the interface accept and return requests and responses, respectively.

TODOs for this PR:

• convert the examples to the new interface
• maybe get rid of the Types module in favor of something else.