* Attempt to build against BoringSSL in CI

* Check for BoringSSL in the SSL bindings

* Check for BoringSSL in the err bindings

* Check for BoringSSL in the pkcs7 bindings

* Check for BoringSSL in the bignum bindings

* Check for BoringSSL in the EVP bindings

* Check for BoringSSL in the X.509 verify bindings

* add PKCS7_get0_signers(), X509_STORE_set_purpose(), and X509_PURPOSE_*

* removed argument names of X509_STORE_set_purpose() and PKCS7_get0_signers()

* Update pkcs7.py

* Update x509_vfy.py

Co-authored-by: Paul Kehrer <paul.l.kehrer@gmail.com>

* PKCS7SignatureBuilder now supports three serializations

PEM, DER, and SMIME. SMIME embeds the S/MIME headers and has the
detached signature concept.

* thanks libre

* smime signer support

* fix ed25519 check

* change some wording

* python 2.7...

* review feedback

* s/secure/signed

* do some verification in the tests

* review feedback

* doc return value

* add pkcs7/smime bindings

* Update src/_cffi_src/openssl/pkcs7.py

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

* Update src/_cffi_src/openssl/pkcs7.py

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

* shrink bindings more

* readd a binding we do need

* readd two more bindings we need

* No more FUNCS/MACROS distinction

* change the docs to not talk about MACROS since they're gone

* remove out of date comment