Copy-on-write wrapper (#28)

* copy-on-write wrapper for containers

* alloc-aware part 1: initial allocator support

* add temp test case

* alloc-aware part 2: everything else

* remove temp test file

* fix support for cows that don't use allocators

* Formatting nitpicking

* Rename `traits_t` to `traits_type`

* some tweaks

* rename to viewed_type and owned_type

* use compressed_pair

* `default_allocator_t` -> `default_allocator_type`

* split into cow_span and cow_writable_span

---------

Co-authored-by: Brady <zeromemesdev@gmail.com>

Author: brampedgex

include/libhat/concepts.hpp

@@ -42,4 +42,7 @@ namespace hat::detail {
 
     template<typename T>
     concept char_iterator = std::is_same_v<std::iter_value_t<T>, char>;
+
+    template<typename T>
+    concept non_const = !std::is_const_v<T>;
 }