Migrate from `hdwallet` to `bip32`

[str4d]

As part of this, we migrate to secp256k1 0.27. This version does not bump secp256k1-sys, so remains compatible with the libsecp256k1 revision used in zcashd.

The zcash_primitives::legacy::keys::AccountPrivKey encoding also changes to preserve the transparent extended key metadata. Previously the type was documented as such, but only encoded the private key and chain code; the new encoding now matches the documentation. As a side effect, the unstable encoding of zcash_keys::keys::UnifiedSpendingKey also changes.

Closes zcash/librustzcash#1407. Closes zcash/librustzcash#1408.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.88889% with 7 lines in your changes missing coverage. Please review.

Project coverage is 63.16%. Comparing base (5a2a670) to head (f54ee4a).

Files	Patch %	Lines
zcash_client_sqlite/src/error.rs	0.00%	3 Missing :warning:
zcash_client_sqlite/src/wallet/init.rs	0.00%	2 Missing :warning:
zcash_keys/src/keys.rs	66.66%	1 Missing :warning:
zcash_primitives/src/legacy/keys.rs	98.14%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1421 +/- ## ========================================== + Coverage 63.14% 63.16% +0.01% ========================================== Files 126 126 Lines 14719 14729 +10 ========================================== + Hits 9294 9303 +9 - Misses 5425 5426 +1 ```

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