Add Processor#merge which allows merging schemas

[solnic]

Refs dry-rb/dry-validation#593

This adds merge interface to all schema types. It allows merging two schemas into a new schema that will include all rules from both schemas as well as their types, steps and parents.

require 'dry/schema'

s1 = Dry::Schema.define do
  required(:name).filled(:string)
end

s2 = Dry::Schema.define do
  required(:age).value(:integer)
end

schema = s1.merge(s2)

puts schema.(name: '', age: 'foo').errors.to_h.inspect
# {:name=>["must be filled"], :age=>["must be an integer"]}