Slicing schemas with partial values

additional schema transformation function, which takes in an value and a schema and returns a new Schema, which is a sub-schema of the original, having all the fields that we not present in the value.

(s/defschema Order
  {:id s/Str
   :type (s/enum :take-away :normal)
   :address {:street s/Str
             :zip s/Str}})

(st/sub-schema 
  {:id "123", 
   :address {:street "Aleksis Kiven katu 24 B 37"}})
; => {:type (s/enum :take-away :normal), :address {:zip s/Str})

(st/sub-schema 
  {:id "123", 
   :type :take-away 
   :address {:street "Aleksis Kiven katu 24 B 37"
             :zip "33200"}})
; => {}

Would be utterly cool with data-driven forms.

Start by locating the schema transformation code and the st/sub-schema entry point referenced in the issue, then compare its behavior with the s/defschema examples. Done means a function accepts a value and schema and returns the original schema with fields present in the value removed, including nested fields, producing the two shown results.