Introduce :zen/type

[niquola]

Users are not good with :zen/tags - we can introduce :zen/type to simplify relationships.
:zen/type should be searchable by zen-tags (probably materialized into zen-tags)

[niquola]

For example

create-pt 
 {:zen/tags #{http/create}
  :operation http/create
  :schemas #{model/patient}
  :response {
    201 {:confirms #{model/patient}}
    422 {:confirms #{http/error}}}}

;; can be

create-pt 
 {:zen/type http/create
  :schemas #{model/patient}
  :response {
    201 {:confirms #{model/patient}}
    422 {:confirms #{http/error}}}}