Either flatten crypto_wrappers or add BoringSSL bindings

[waywardgeek]

The noise_handshake code from the Chromium folks used an abstraction layer around the crypto operations to allow use of Ring crypto, BoringSSL Rust bindings, or Rust crypto. For the initial PR, all but the Rust crypto were deleted, but they were not inlined into the files calling these APIs.

These APIs are Ring-centric and should be cleaned up to be more generic and simpler to use, if we want to support bindings besides Rust crypto. If we do want to support other bindings, we may want to widen this layer so it can be used elsewhere, such as the Oak Restricted Kernel, etc. If not, these APIs should be inlined into the two files that use them.