Introduce a ProtoBoeuf::CodeGen::Field to alleviate feature envy (#181)

* Introduce a DecoratedField to alleviate feature envy

* Rename DecoratedField to ProtoBoeuf::CodeGen::Field

* We want to test map_type, not map_field? here

* This rule needs to be more specific.  lib/protoboeuf/codegen.rb was getting caught by it.

* assert_predicate/refute_predicate is a better fit here.

* Sort these delegators lexographically

Author: davebenvenuti

Rakefile

@@ -74,7 +74,7 @@ rule ".rb" => ["%X.proto"] + codegen_rb_files do |t|
   File.binwrite(t.name, ProtoBoeuf::CodeGen.new(unit).to_ruby(dest, options))
 end
 
-rule ".rb" => "%X" do |t|
+rule %r{lib/protoboeuf/google/.+\.rb} => "%X" do |t|
   # Given lib/protoboeuf/google/protobuf/foo.rb and lib/protoboeuf/google/protobuf/bar.rb, generate
   # lib/protoboeuf/google/protobuf.rb that looks like:
   #
@@ -132,7 +132,7 @@ end
 
 Rake::TestTask.new do |t|
   t.libs << "test"
-  t.test_files = FileList["test/*_test.rb"]
+  t.test_files = FileList["test/**/*_test.rb"]
   t.verbose = true
 end