CMake'ify the repository

[eyalroz]

Even though this is a single-header library - it's still useful to have a CMake for it. Benefits:

1. Plays well with testing, especially when TODO: Add proper tests #4 happens. Particularly,
2. Makes it easier(ish) to build with different compilers on different platforms
3. Allows installation in a way which other projects can use find_package(inplace_vector) and then target_link_libraries(myapp inplace_vector::inplace_vector) just like any library.
4. Package managers like conan and vcpkg like it. In case you plan on releasing.

[TedLyngmo]

100% agree and support this if you'd like to give it a go!