Network checking on `DescriptorPublicKey` and `DefiniteDescriptorKey`

[apoelstra]

When working on descriptor support for stratum I realized that I would like to check for testnet keys and (attempt to) refuse to use them on mainnet, on the basis that mainnet coins sent to testnet addresses may wind up blackholed.

This isn't too hard for me to implement by iterating through all my DescriptorPublicKeys, looking at the xpubs (if they are xpubs) and looking at the network bytes.

But this raises a few questions:

• At parsing time should we insist that xkeys have consistent network bytes?
• Should we expose some sort of convenience method for obtaining these bytes?

I am thinking about adding an enum

enum XKeyNetwork {
    /// No xkeys are present
    NoXKeys,
    /// xkeys are present but have conflicting network prefixes
    Mixed,
    /// xkey(s)s are present and all have the same network prefix
    Single(bitcoin::Network),
}

and a helper method on Descriptor<DescriptorPublicKey> and Descriptor<DefiniteDescriptorKey> that returns this enum.