Resolve -Wsign-conversion warnings

More CMake fixes...

More CMake contributions and official deprecation of CMake support.

A minor release that fixes an issue with CMakeLists.txt: #115

A minor release that contains a fix for CMakeList.txt by @ufleisch

Fix for #111 - broken utf8::unchecked::utf16to8()

This is a major release that includes the following new features:

• Utf16 strings codepoint iteration and appending: #91 (special thanks to @ceztko)
• Support for u8string (thanks to @XCI9)
• find_invalid and is_valid for C-style strings

There was also a complete overhaul of the CMake build system - testing is now not included in the main CMakeList.txt file and is not a part of the "build".

Fix for #78 . Documentation updates. More CMake contributions.

Note that this release contains a bug described in #111 . If you use unchecked namespace, please avoid it.

Minor CMake improvements.

A minor release that includes a fix for https://github.com/nemtrif/utfcpp/issues/92[: Clang 10 Compiler warnings](e3e57c8) and build system improvements by @Finkman