Containers: add StringView overload for known sizes

[sthalik]

Is this a reasonable compromise between support for unterminated strings and the need for a constexpr constructor?

[sthalik]

Actually this isn't a good idea. I forgot about you mentioning operator""_s.

[mosra]

Just for the record, as we discussed this on Gitter extensively:

I plan to have a constexpr constructor relying on compiler-specific magic, such as the one that's used to implement constexpr construction of std::string_view. But, due to the compiler-specific magic, only as an optional "nice to have" optimization that removes the need for runtime strlen() even when _s literals are not used, not something people should rely on.

For constexpr creation from arrays I plan to update the ArrayView construxctor to be constexpr, as detailed in #123, when I figure out how without adding a header dependency.