After #561 (comment) and #561 (comment)

1. Leave bitcoin::PublicKey as is, probably adding bitcoin::util::ecdsa::PublicKey alias to it (the same for private keys).
2. Re-export Taproot keys under bitcoin::util::schnorr
3. Change ExtendedP(ub/priv)Key to hold secp256k1::Key rather than bitcoin equivalents. It is a necessary change anyway, since BIP32 does not support uncompressed keys and using type with compression flag is a mistake
4. Introduce to Extended*key* functions to_ecdsa (returning always compressed ECDSA key) and to_shchnorr.
   ... and that's all for the first non-API breaking release.

Next, more invasive change will be:

1. Remove bitcoin::PublicKey type
2. Create UncompressedKey types (for legacy purposes) and
3. Introduce PublicKey enum with just Compressed and Taproot variants (such that it can be used in SegWit miniscript/descriptor context without the need for errors/panics).

The first phase is implemented with the following PRs:

• Non-API breaking changes for 0.26.1 Non-API breaking introduction of Schnorr keys #589
• API-breaking changes to Extended keys internal key representation: Taproot: BIP32 extended keys using Scep256k1 keys instead of bitcoin ECDSA #590
• API-breaking changes to PSBT internal BIP32 key representation: PSBT BIP32 keys using to Secp256k1 keys instead of bitcoin ECDSA #591
• Minimally-invasive separation of bitcoin keys from ECDSA signature types #757