Use nerf gun

@othiym23 @isaacs 

Supercedes #6514. Corresponding fix for https://github.com/npm/npm-registry-client/pull/77. Moved the duplicated `toNerfDart` implementation across `npm/npm` and `npm/npm-registry-client` to [nerf-gun] and applied the appropriate fix.

[nerf-gun]: https://github.com/indexzero/nerf-gun