Multiple changes.

- Remove the runtime dependency on rake-compiler.
 - Use #rb_str_new instead of #rb_str_new2, the hash character array
   isn't null terminated.
 - Add various safeguards for object instantiations.
 - Remove the 'threaded' options for messages, folding them into
   'archived'.  If archiving is enabled, so is threading.
 - Return nil for lookups from the list object instead of raising
   exceptions.
 - Open subject indexes with the proper encodings (thanks Michael
   Granger!)
 - Allow touching and unlinking files to operate on multiple paths
   at once, within a single safety() wrap.

Rakefile

@@ -59,7 +59,7 @@ environment.)
 	s.required_ruby_version = '>= 2.1'
 
 	s.add_dependency 'mail', "~> 2.6"
-	s.add_dependency 'rake-compiler', "~> 1.0"
+	s.add_development_dependency 'rake-compiler', "~> 1.0"
 end
 
 Gem::PackageTask.new( spec ) do |pkg|