Describe Malli Schema Children using Malli

[ikitommi]

Each Schema has it's own syntax for children. We should describe them using Malli to get proper syntax validation. This is just a new protocol method in IntoSchema. Need to be an option for the DCE to remove these at runtime. This requires #180 to be implemented.

[miikka]

Example by @mvarela – this broken schema is quietly accepted, but ideally it would give a syntax error:

;; Broken: :version schema is accidentally included in :project vector
(def Project [:map [:project string? :version string?]])
(m/validate Project {:project "malli", :version nil})  ; => true

;; The correct version, for comparison:
(def Project [:map [:project string?] [:version string?]])
(m/validate Project {:project "malli", :version nil})  ; => false