IntoSchema knows type and schemas for properties and children

[ikitommi]

Related to #269 & #270. Move methods from Schema to IntoSchema so that we can describe Malli schemas with Malli.

(defprotocol IntoSchema
  (-type [this] "returns type of the schema")
  (-type-properties [this] "returns schema type properties")
  (-properties-schema [this] "maybe returns :map schema describing schema properties")
  (-children-schema [this] "maybe returns sequence schema describing schema children")
  (-into-schema [this properties children options] "creates a new schema instance"))

the print-method now shows the :type:

(m/-enum-schema)
; => #IntoSchema{:type :enum}

(m/schema [*1 "A" "B"])
; => [:enum "A" "B"]

(type *1)
; => :malli.core/schema