Implement hdkeychain BIP0032 API.

This pull request adds a new sub-package named hdkeychain which can be used to derive hierarchical deterministic key chains which form the foundation of HD wallets.

- Support for private and public extended keys
- Convenient cryptographically secure seed generation
- Simple creation of master nodes
- Support for multi-layer derivation
- Easy serialization and deserialization for both private and public
  extended keys
- Support for custom networks by registering them with btcnet
- Obtaining the underlying EC pubkeys, EC privkeys, and associated bitcoin addresses
  ties in seamlessly with existing btcec and btcutil types which provide
  powerful tools for working with them to do things like sign transactions
  and generate payment scripts
- Makes use of the btcec package which is highly optimized for secp256k1
- Code examples including:
  - Generating a cryptographically secure random seed and deriving a
    master node from it
  - Default HD wallet layout as described by BIP0032
  - Audits use case as described by BIP0032
- Comprehensive test coverage including the BIP0032 test vectors
- Benchmarks
The Neuter method doesn't actually modify the private extended key, it only returns a new instance of a public extended key.

There's also 2 "be"s in the first sentence here.