add ec.private_key_from_secret_and_curve (#3225)

* finish https://github.com/pyca/cryptography/pull/1973

* change API & add test

Function will now return an instance of EllipticCurvePrivateKey, as that
is the users' ultimate goal anyway.

* fix test

* improve coverage

* complete coverage

* final fix

* centos fix

* retry

* cleanup asserts

* use openssl_assert

* skip unsupported platforms

* change API name to derive_private_key

* change version added

* improve description of `secret` param

* separate successful and failure test cases

* simplify successful case

* add docs for derive_elliptic_curve_public_point

* add period
8 files changed