Decide on a re-export policy for zebra-chain

[hdevalence]

In #199 I introduced an inconsistency; zebra-chain reëxports redjubjub but not ed25519-zebra. This is a pretty minor issue (just where stuff should go in the tree and how) but it would be good to have a consistent answer to the question of "when should zebra-chain reëxport dependencies".

One possible answer would be to do it only for crates developed by the ZF that are supposed to be part of zebra-chain, but are actually separate crates in order to allow reuse by zcashd or librustzcash. This would include both redjubjub and ed25519-zebra as well as a standalone crate for note encryption (#181).

Another answer would be to reëxport any crate or type which appears in public API for zebra-chain.

[dconnolly]

We're going to leave this open for a little while longer to see our dependencies and crates shake out.

[dconnolly]

"If it's part of your api, you should export it." So we need to export ed25519-zebra. #405