Added curve P-521 (aka secp521r1).

[pupeno]

[jsmestad]

:+1:

[earonesty]

lgtm

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}