This library defines two string helper classes.

# CowStr

Is a String that can be initialized to a static string or be build dynamically. Its contents
can then be immutably shared. When mutation is required the string will be copied first.


# SubStr

Refers to a (immutable) span inside a `CowStr`.


# Features

 * **singlethreaded** uses `std::rc::Rc` for sharing strings. Default is `std::sync::Arc`.
