Replace native-tls with rustls

[lukateras]

native-tls seems to be one of the few remaining native dependencies. It wraps platform-specific TLS implementations (schannel on Windows, Security framework on macOS, OpenSSL on Linux).

There are several problems with that:

1. All mentioned TLS implementations are memory unsafe.
2. Native dependencies make compilation more complex, especially cross-compilation, especially cross-compilation to static executables.

It seems that net crate imports native-tls for reqwest. If that's the case, reqwest optionally supports rustls, pure Rust TLS implementation: seanmonstar/reqwest#378

Advantages:

• Superior performance [1] [2] [3] [4]
• Memory safety
• Trivial to (cross-)compile

Disadvantages:

• No old TLS versions, fewer ciphers implemented (focus of rustls is on safe defaults): OTOH, for holochain-rust use case, this is probably a good thing, and makes downgrade attacks less effective
• Did not go through rigorous security audit (although attack surface is smaller)

[lukateras]

cc @thedavidmeister

[lukateras]

In particular, this currently blocks us from building for aarch64-unknown-linux-musl (statically compiled AArch64 releases), issue seems to be rooted in openssl-sys (neither vendored nor normal builds work for different reasons).