Add a namespace to Unicode Inflection functions

[grhoten]

The following issue was mentioned with the recent Unicode MessageFormat integration.

Un-namespaced function identifiers like :inflect and :quantify are reserved for the MF2 spec's own use. We have also reserved the "u" namespace, which might be applicable for this? The functions would then be called as :u:inflect and :u:quantify. Using an inflection-specific namespace would also be appropriate. If the reservation of these identifiers was not clear from the spec, we'll need to fix things.