Implement to_json(json : JSON::Builder) on BigDecimal

[petr-fischer]

Implement method to_json(json : JSON::Builder) on BigDecimal class. BigInt and BigFloat are OK. to_yaml(...) is properly defined in the file "yaml/to_yaml.cr" on the Number superclass, to_json can be probably defined on the Number class too (in the file "json/to_json.cr").

[petr-fischer]

Simple dirty fix is to add this code:

struct BigDecimal
  def to_json(json : JSON::Builder)
    json.raw(self.to_s)
  end 
end

to the big/json.cr. But I personally don't like it, because YAML treatment of BigDecimal is probably more "elaborated".