Clang does not like that we are trying to refer to our own size while
our declaration is not yet complete, and fails to compile this file.
This is fixed by introducing a function which returns the correct
sizeof. This only gets evaluated in the `requires` clause after the
whole class has been parsed, so it will compile fine.
Doing these as custom classes might be faster, especially when writing
them in SSE, but this would cause a lot of Code duplication and due to
the nature of constexprs and the intelligence of the compiler they might
be using SSE/MMX either way
