switch truncate_digest_for_ecdsa to context manager

Also adds a docstring to fix #1335
Merge pull request #1349 from reaperhulk/more-ctx-mger

switch truncate_digest_for_ecdsa to 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
extraneous blank line excision