Well, there should be a Standards.URI()->encode() too as Mirar suggested, but that's a different matter.
If we had real static functions it'd of course be simplest to add one in Standards.URI, but I don't think it's really the right place for it.
Perhaps a way to go is to move the module entirely? I'm not particularly fond of the "Standards" category at all. It's too vague and overlaps with other things. (I mean, http is a standard, so why not Standards.HTTP? MIME is a standard and yet it's at the top level.) Same goes for the "Tools" directory, for that matter.
I favor a flat namespace, and when it comes to well known acronyms like "URI" I don't see any problem with having it directly at the top level. So my suggestion is to move Standards.URI to URI.URI and add URI.encode_string.