Associated constant for to_int

[ExpHP]

Once Rust implements associated constants, an associated constant will replace the
function to_int().

This feature has landed! 🎉

(by the way, despite what the toplevel docs say, this crate isn't entirely useless; there's no better representation for indexing an HList!)

[paholg]

So it has. I have just recently implemented it in v1.10 of typenum. I would accept a PR to add associated constants here, but am unlikely to ever do it myself.