Backporting `ssh-key` bug fixes to 0.6.x

[mkeeter]

I've backported #289 and #291 to ssh-key/v0.6.6, and updated the CHANGELOG:

https://github.com/oxidecomputer/SSH/commits/ssh-key/v0.6.6-backports/

It would be great to have this released as 0.6.7, so that we don't have to use a git dependency while waiting for the 0.7.0 release. In particular, #291 is causing our CI runs to occasionally fail, since picking an undersized mpint is a matter of bad luck.

Thanks!

[steveklabnik]

Hey @tarcieri, any chance you have a moment to take a look at this?

[tarcieri]

@mkeeter can you open a PR and target the stable branch?

[tarcieri]

Released in v0.6.7 (#302)

[mkeeter]

Thanks!