Remove global side-effects from mw.ustring library registration

The point of putting the unicode library in mw.ustring instead of
ustring was to avoid conflicts with future upstream work, and with other
libraries. It rather defeats the purpose if you then modify the global
string table during module startup.

Users can always set up local aliases if they feel "mw.ustring" is too
much to type.

Change-Id: I5d0ddb70d999aeb6e36e6ddbcdb19922d0274a39
Fix further non-local effects of library registration

Fixed several accidental leaks to the global namespace due to missing
"local" declaration. Removed extension of the string table by mw.uri,
same justification as I5d0ddb70.

Change-Id: Iba1bf8e651d4ce05812e4a9a7a074cb6679297a0