Fix deprecation warning

Also put iterator into dhandy namespace. I don't
remember why I left it out, and it's probably a
sign I should have not done so.

test/unit/meson.build

@@ -9,6 +9,7 @@ unit_test_prog = executable(meson.project_name(),
   'resource_pool_test.cpp',
   'version_test.cpp',
   'tiger_test.cpp',
+  'infix_iterator.cpp',
   install: false,
   dependencies: [sprout_dep, catch2_dep],
   include_directories: [public_incl],