Add support for ChaCha20 with LibreSSL

[facutuesca]

This PR adds support for ChaCha20 with LibreSSL. Since cryptography's ChaCha20 uses OpenSSL's implementation (which uses a 64 bit counter + 64 bit nonce), and LibreSSL does the same, it is straightforward to use LibreSSL's API.

The only complication is that the current _CipherContext implementation assumes that the underlying cipher can be accessed through the EVP_CIPHER API. This is not the case for LibreSSL's ChaCha20, which has a separate CRYPTO_chacha_20() API for it.

In order to solve that, this PR makes _CipherContext an abstract class with two implementations, _CipherContextEVP (which is the previous _CipherContext) and _CipherContextChaCha, which is a simple context that is used only for the LibreSSL+ChaCha20 combination.

This means all of the code that uses _CipherContext remains the same, and we only have a single branching point isolated in the create_cipher_context() function, which selects the correct context depending on the cipher+backend.

It also leaves the option open in the future if we want to add BoringSSL's ChaCha20, since we can reuse the same _CipherContextChaCha for it (the API is almost the same, and we can deal with the differences in a C wrapper)

[facutuesca]

Needs further work