Copy-on-write wrapper

[brampedgex]

This PR adds hat::cow<T>, a copy-on-write wrapper for dynamically allocated containers.

What's done so far:

• cow<T> and its basic API
• cow_traits specializations for std::basic_string_view, hat::basic_cstring_view, and std::span
• Basic allocator support (default construction only)
• Support allocator args in constructors
• Make the STL know we are allocator-aware

[brampedgex]

rebase