Added CStrRef

This type should take over the no-ownership use case from CString.
Added method with_len for CChars

CChars, besides being an iterator, is a temporary length-unaware
wrapper for a string pointer, so it makes sense to provide a
conversion method to the length-aware CStrRef.
Renamed with_len to measure

In naming conventions of ToCStr and elsewhere, `with_x` is a method
taking a closure to call.