small_vector and small_string are not trivially copiable

[cschreib]

Because of the pointer stored in the basic_small_vector member. This also prevents constructors from being truly constexpr, so these types cannot be created at compile time.

The trick of basic_small_vector was used so that we could implement functions using these classes, without needing a template argument for the maximum size.

There are other ways to achieve this.

One would be to have basic_small_vector be created on the fly rather than stored, and have it hold a pointer to the size (so it can update it). Then it should be renamed small_vector_view.

This is likely required to implement #1.