Integrate BoringSSL source code

[ilammy]

I’m going to start with BoringSSL as the first supported cryptographic backend because it allows to have zero system dependencies. I really like the approach by Google in their Mundane library (a competitor, hehe) so I guess I’ll reuse it.

MIT-licensed code is compatible with Apache License 2.0 so we can integrate the build scripts with no issues. BoringSSL itself is... well, it’s complicated. See their license here. Generally, that’s a mix of BSD-style licenses and MIT which are both compatible with Apache so we can incorporate that as well.

Checklist:

• Take Mundane’s build scripts and put them into soter-boringssl-sys crate (BoringSSL build system #2)
• Use a different prefix to avoid conflicts with Mundane (BoringSSL build system #2)
• Export really few functions at first (e.g., PRNG) (Generate bindings with bindgen #4)
• Add smoke tests for exported unsafe functions (Generate bindings with bindgen #4)
• Configure builds for Linux, macOS, Windows (Test across Linux, macOS, and Windows #5)
• Clarify licensing in the documentation (Document soter-boringssl-sys crate #6)
• Describe build setup in the documentation (Document soter-boringssl-sys crate #6)
• Correct metadata for the soter-boringssl-sys crate (Document soter-boringssl-sys crate #6)

[ilammy]

Pretty much complete in 23 hours, including time to setup some basic CI. I have no life 😭