start and end BN_CTX so BN_CTX can track func owner of temporary BNs
put BN_CTX_end in a finally to ensure it is invoked
extraneous newline murdered
switch to a BN_CTX context manager.
add EC WithNumbers interfaces and docs
Support EC WithNumbers on OpenSSL backend + tests
fix naming mistake in the docs (for DSA and the new EC docs)
refactor to use ctx manager and share logic with set_public_key_affine