Support for importing raw key bytes

[BHawleyWall]

Hello - your implementation of slip-0010 is one of the few "in the wild" for Rust. I recently had need for implementing HD-keys in Rust and after exploring the options available, I liked your crate the most out of the options available. I was able to put together a decent implementation that was clean and readable thanks to your abstractions and was successful in generating and extending keys.

Unfortunately, I've reached a point where I now want to support extending keys (deriving some new step or steps, not from a seed) generated from other systems. Your ExtendedSecretKey and ExtendedKeyPair types do not seem to expose any constructors from a slice of bytes for the secret material and chain code.

Was this intentional, or would it be possible to request a way to "import" an ExtendedSecretKey from just the raw bytes for the key + chain code?