v0.3.0

This was a decently large refactor with some important security changes (rollback) and better documentation.

• The handshake state will rollback its internal SymmetricState if read_message() or write_message() failed for any reason, so the state will not be left in a weird or potentially dangerous state.
• NoiseBuilder is now just Builder.
• All errors returned in snow are now a SnowError enum type.
• generate_private_key() is now generate_keypair() in the builder.
• Crypto types must implement Send + Sync now.
• The default resolver is now optional if you implement your own resolver and didn't want it.