feat(sec1): add default cargo feature for disabling point module

[bstrie]

This allows users who aren't using EncodedPoint to omit dependencies on generic-array, typenum, and version_check.

[tarcieri]

You'll need to enable the point feature inside of the ssh-key crate

[bstrie]

@tarcieri Fixed. I find it quite disorienting that a cargo test in the top level directory would succeed while a cargo check in the ssh-key directory would fail.