make runtime key inference happen in I18n.normalize_keys

[jenseng]

every time I18n.translate is called without a key, it is inferred from the default (unless you're using the literal default as the key). really this should only happen once per call site, since the default doesn't change.

rather than have our own cache, we should extend I18n.normalize_keys so we can take advantage of I18n's existing normalized_key_cache.

bonus: if someone is using I18nema, then our inferred keys can live outside of Ruby-land for GC happiness