Move some .rake files to tasks directory for consistency

src/mruby_core.rake

@@ -1,17 +0,0 @@
-as_cxx_srcs = %w[vm error gc].map{|name| "#{MRUBY_ROOT}/src/#{name}.c"}
-
-MRuby.each_target do
-  objs = Dir.glob("#{MRUBY_ROOT}/src/*.c").map do |src|
-    dst = src.pathmap("#{build_dir}/src/%n")
-    if cxx_exception_enabled? && as_cxx_srcs.include?(src)
-      compile_as_cxx(src, "#{dst}.cxx")
-    else
-      objfile(dst)
-    end
-  end
-  self.libmruby_objs << objs
-
-  file libmruby_core_static => objs do |t|
-    archiver.run t.name, t.prerequisites
-  end
-end