Make void a generic BindingIdentifier

[bergus]

The proposal currently only allows using void = … as a special form of the UsingDeclaration, adding a special case [+Using] void Initializer to the LexicalBinding production.

I think it would be more consistent to allow void in any place where a BindingIdentifier is allowed. This would allow a lot of useful patterns where currently _ is often used to indicate an unused variable (or _unused1, _unused2 if multiple bindings were to be declared):

const void = …;
let void = …;
var void = …;
using void = …;
for (const void of …) {…}
const {x: void, y: void, ...rest} = …;
const [void, void, ...rest] = …;
function …(void, second) {…}
onClick = (void) => {…};

[rbuckton]

This is a duplicate of #77, and I've already commented as to my position in that issue.