Move to nested handshake message structure

Having the nested structure:

    Handshake Message:
        Noise part (zerocopy message)
        MAC footer part (zerocopy message)

Greatly simplifies processing the MAC fields,
since the MAC footer covers the noise part, which can
be accessed as bytes using AsBytes.

Cargo.toml

@@ -15,6 +15,11 @@ rust-crypto = "^0.2"
 generic-array = "0.12.3"
 zerocopy = "0.2.7"
 byteorder = "1.3.1"
+digest = "0.8.0"
 
 [dependencies.x25519-dalek]
 version = "^0.5"
+
+[dependencies.subtle]
+version = "2.1"
+features = ["nightly"]