* Made string_span details::string_length() generic
removed overloads & specialized classes
Creating string_spans using `char16_t` and `char32_t` was not possible
without creating new specializations and function overloads.
This patch makes details::string_length() generic removing the need to
extend the overloads and specializations.
* added type aliases for string_span types char16_t and char32_t
* Added char16_t & char32_t overloads for ensure_z
* added string_span tests for char16_T & char32_t
* added zstring type aliases for char16_t & char32_t
* Added tests for char16_t & char31_t zstring and string_span types
* applies clang format to <gsl/string_span>
* Clang format tests/string_span_tests.cpp
* Removed ensure_z() overloads as they don't add functionality.
Galik
4c5fdb541f