Initialize structs

Initialize a Go struct wrapping a C type by calling the C initialization function before use, as the zero value is invalid for C libraries like GMP. In the provided gmp package, the doinit method checks an init flag and calls C.mpz_init on the underlying C pointer if the struct hasn't been initialized yet.

func (z *Int) doinit() {
	if z.init {
		return
	}
	z.init = true
	C.mpz_init(&z.i[0])
}

Always call doinit (or a similar wrapper) at the start of any method that accesses the C struct fields to prevent crashes from using uninitialized C memory.

When Go wraps a C library, the default empty value (zero value) often doesn't work because C requires manual setup. You must run a specific setup function once before using the object, similar to turning on a machine before pressing buttons. This code checks if the setup happened and runs it if needed.