Resolve copyright notice issue

[daira]

The librustzcash crates have copyright notices in the MIT license file that say only "(c) <date> Electric Coin Company" (see for example here and the discussion here). However, it has been our longstanding practice to accept external contributions to MIT or MIT/Apache licensed code without assignment of copyright to ECC.

(Apache license files do not include the copyright holder(s). This should probably be considered an independent bug, although users are likely to reasonably infer that the copyright holder(s) are the same when relying on the Apache license as when relying on the MIT license.)

IMHO the copyright notices should be the same as on zcashd code, i.e. they should also include "(c) <date> The Zcash developers"; see https://github.com/zcash/zcash/blob/master/COPYING and https://github.com/zcash/zcash/blob/master/contrib/debian/copyright. (Aside: sometimes this notice is missing from per-file notices on zcashd code inherited from Bitcoin but modified in Zcash, but that is a bug: zcash/zcash#2921.) There should also be an automatically updated CONTRIBUTORS file in each repo, as there is for zcash/zcash.

Note that retaining the existing notice and requiring copyright assignment to ECC is not really an option unless we obtain agreement to the assignment from all previous contributors to librustzcash crates. I think that seeking such agreement would have a significant risk of causing (arguably justified) ill feeling and suspicion, and is not a good idea.