* enable wconversion and finish fixes

* don't pass -Wconversion if it's win32

CRYPTOGRAPHY_SUPPRESS_LINK_FLAGS will now suppress link flags regardless
of platform. Additionally, CRYPTOGRAPHY_WINDOWS_LINK_LEGACY_OPENSSL is
now the flag you need if you want to link against < 1.1.0 on windows.

* First pass at adding bindings for CT functions. No conditionals yet.

* add a stack typedef as well

* Don't try to include this header if we're on an older OpenSSL

* wire up the conditional stuff

* bunch o' nonsense to get it to compile on old openssl

* I hate libressl

* support defining which windows libraries to link with an env var

CRYPTOGRAPHY_WINDOWS_LIBRARIES is your new friend

* add some docs

* change to CRYPTOGRAPHY_WINDOWS_LINK_OPENSSL110

* lib prefixing is not a thing msvc does, right

* New osrandom_engine in C

Inspired by Python/random.c and the old implementation.

Signed-off-by: Christian Heimes <christian@python.org>

* osrandom_engine

* Fix naming bug caused by search 'n replace mistake
* Make it easier to override osrandom auto-detection
* Add engine ctrl and backend API to get implementation from ENGINE

Signed-off-by: Christian Heimes <christian@python.org>

* Better test coverage, documentation, LICENSE

Signed-off-by: Christian Heimes <christian@python.org>

* Coverage is hard.

Signed-off-by: Christian Heimes <christian@python.org>

* * enable win32 check
* read() returns size_t

Signed-off-by: Christian Heimes <christian@python.org>

* Add macOS to spelling list. Remove dead code from header file.

Signed-off-by: Christian Heimes <christian@python.org>

* remove CCRandomGenerateBytes path and update getentropy to work on macOS

This change allows us to test all the engines in our CI:
* getentropy (tested by macOS sierra)
* getrandom (tested on several linux builders)
* /dev/urandom (tested on FreeBSD, OS X 10.11 and below, & older linux)
* CryptGenRandom (tested on windows builders)

I also fixed bugs preventing compilation in the getentropy code

* getentropy() returns int and is restricted to 256 bytes on macOS, too.

Signed-off-by: Christian Heimes <christian@python.org>

* add versionadded

* Re-add import of os module

* Fixes related to Alex's recent review.

Signed-off-by: Christian Heimes <christian@python.org>

* Add error reporting and fail for EAGAIN

Add error reporting strings for various error cases. This gives us much
nicer and understandable error messages.

SYS_getrandom() EAGAIN is now an error. Cryptography refuses to
initialize its osrandom engine when the Kernel's CPRNG hasn't been
seeded yet.

Signed-off-by: Christian Heimes <christian@python.org>

>>> lib.EVP_get_digestbyname(b'md_gost94')
<cdata 'EVP_MD *' NULL>
>>> lib.OPENSSL_config(ffi.NULL)
>>> lib.EVP_get_digestbyname(b'md_gost94')
<cdata 'EVP_MD *' 0x10adc7440>

* Use a series of constants for OpenSSL version checks.

N.B. I removed several qualifiers that were being used to express beta vs. release in OpenSSL version numbers. Reviewers please look closely!

* Convert some python as well, also add the file

* flake8

* Simplify code, remove functionality that can be expressed more simply

* clean up the tests as well

* more constants

* wrap long lines

* reflect feedback

* unused

* add this back?