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`. Since `SubStr` are always immutable they store
the referred span directly which makes dereferencing them faster than `CowStr`.
# Features
* **singlethreaded** uses `std::rc::Rc` for sharing strings. Default is `std::sync::Arc`.