added Protobuf::Message#[]=(key,val), Protobuf::Message#to_hash

[maiha]

This PR adds #[]= and #to_hash those provide runtime meta programming.

usecase of []=

module Protobuf::Message
  def merge(hash)
    hash.each do |k,v|
      self[k] = v
...

usecase of to_hash

require "json"

struct User
  include Protobuf::Message
  contract_of "proto2" do
    required :user    , :string, 1
    optional :password, :string, 2
  end
end

record FilterParameter, keys : Array(String) do
  def apply(hash)
    hash.dup.tap{|h|
      keys.each{|k| h[k] = "[FILTERED]" if h.has_key?(k)}
    }
  end
end

pb = User.new("maiha", "1234")
pb.to_hash         # => {"user" => "maiha", "password" => "1234"}
pb.to_hash.to_json # => {"user":"maiha","password":"1234"}

secure = FilterParameter.new(["password"])
secure.apply(pb.to_hash) # => {"user" => "maiha", "password" => "[FILTERED]"}

Best regards,