Add `Utf8`/`Utf16` methods for non-zero terminated strings

I find it odd that this package only provides support for null-terminated strings with the `Utf8` and `Utf16` marker types.

It's quite common these days to not use zero terminated strings, but to use string views which are ptr + size. This is done for example in Rust (`&str`) and C++ (`std::string_view`). Advantages over null-terminated strings are constant time length and no buffer overflows if you lose the `\0`.