`val (!!) : string -> ustring`

Prefix operator that converts primitive string to `ustring`. May raise some exceptions depending on actual strings implementation.

You might want to `open Path.OperatorLift` to improve readability of path construction using string literals. Example: `Path.root/:!!"foo"/:!!"bar"` = `Path.root/:(S.lift "foo")/:(S.lift "bar")` (where `S.lift` converts to `ustring` type)