Add use_cases.cpp
Remove "operator value_type() const noexcept" from buffers. Make the buffer ctor explicit. Buffers were requested by people who do not want automatic conversions because they want full control over conversions, and are very concerned about "efficiency".

Add "operator value_type() const noexcept" to the endian arithmetic types. These types are used by people who do want fully automatic conversions. The arithmetic types need to be drop-in replacements for the built-in arithmetic types for these users.