fix(crystal): allow users to YAML.dump(model) to ease debug

Author: n-rodriguez

modules/openapi-generator/src/main/resources/crystal/model.mustache

@@ -2,6 +2,7 @@
 
 require "big"
 require "json"
+require "yaml"
 require "time"
 
 module {{moduleName}}

modules/openapi-generator/src/main/resources/crystal/partial_model_generic.mustache

@@ -3,6 +3,7 @@
   {{/description}}
   class {{classname}}{{#parent}} < {{{.}}}{{/parent}}
     include JSON::Serializable
+    include YAML::Serializable
 
     {{#hasRequired}}
     # Required properties