Separate `utils` (if any) and `extensions`

[dninemfive]

Currently, all classes which implement extension methods are in a utils namespace. The name of the project is utl because everything in here is a utility, so these should be in a separate, more-precisely-named extensions namespace, and have the classes renamed to match.

Anything in utils which is not an extension method (i am currently refactoring many of these anyway) should remain in a utils namespace for now.