Is the `zero` instance member still required?

[coriolinus]

counter-rs/src/lib.rs

Lines 293 to 294 in 9e217dc

	// necessary for `Index::index` since we cannot declare generic `static` variables.
	zero: N,

That code was added in #9 (see discussion) and has never sat quite right. Can we work around it now? Leak exactly one zero item for each N? Bypass it some other way?