make variant ctor constexpr (felixguendling#125)

* make variant ctor constexpr

* make ~variant constexpr only for c++20 and up

Pretty Printers for Cista (felixguendling#107)

add license to single file version

Win32 CI (felixguendling#69)

bugfix release

Version 0.5

Update README.md

introduce namespaces: cista::offset and cista::raw

namespaced deserialize functions, unchecked_deserialize

add support for offset based serialization

offset_ptr: stores the offset as difference between `this` and the object it points to
Serializing this allows to skip the deserialization step and just use a `reinterpret_cast<T>(buf.begin())`.
This is useful for example in situations with shared memory.

fix msvc debug