CLASS_SIGNALMAININ should ensure that the variable has the correct type/size

[Spacechild1]

The CLASS_SIGNALMAININ macro implicitly expects a variable of type t_float. If you accidentally pass a variable of the wrong type (e.g. double in single-precision Pd), the compiler silently accepts it and you get garbage at runtime.

With C11 we could actually check the type/size with _Static_assert (https://en.cppreference.com/w/c/language/_Static_assert). I think modern toolchains default to C11< anyway, but we can use conditional compilation to keep C89 compatibility.

Also, we could use offsetof (https://en.cppreference.com/w/c/types/offsetof) to calculate the member offset, instead of the terrible NULL pointer cast trick. In fact, offsetof is part of the C89 standard!