Deprecate Buffer interop in preparation for 2.0.0

In 2.0.0, a major focus is on making the API as simple as possible, and to avoid possible interop concerns.
This is just one of those preparations.

[The specification](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#serialization-format) does mention:

> This 78 byte structure can be encoded like other Bitcoin data in Base58, by first adding 32 checksum bits (derived from the double SHA-256 checksum)

Implying maybe, that you could expose the pure byte format in a standard way, however I feel the checksum is useful in most contexts, and unless a use case can be garnered for the `toBuffer`, I'm happy to remove them in `2.0.0`.