This library defines strings with copy-on-write semantics.

# CowStr

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

This resembles a improved `Arc<Cow<'static, str>>` with some differences:
 - No double dereference going from `Arc` to `String` to the actual data.
 - There are no `Weak` references.
 - `CowStr` can be interior-mutable extended (append at the end).
 - Improved `AllocationStrategy`

`CowStr` implements many of the `std::String` methods. Missing methods will be added as required,
PR's are welcome.


# SubStr

Refers to an immutable slice inside of a `CowStr`. This somewhat resembles the `String`/`&str`
relationship while `SubStr` keeping a strong reference to its orgin `CowStr` and thus don't
need lifetimes.


# Features

 * **serde** wrapping the default string (de)serialization.
 * **nightly** enable some nightly optimizations and extensions


# Implementation Notes

`CowStr` is made for sharing immutable strings, a short single `CowStr` will occupy slightly
more memory than a std `String`. But as soon a `CowStr` is cloned or `SubStr`'s are used
it pays back. Especially since `SubStr` don't need lifetimes as they use reference counting
memory management becomes significantly easier.

Because rust has yet incomplete support for DST's `CowStr` needs some unsafe code. This is
liberally chosen, when possible contracts are enforced by `debug_assert`'s.


# Testing

`CowStr` commes with an extensive test suite. 'cargo-mutants' is used to detect missing
tests. Releases must pass testing under 'miri'.