Merge bitcoindevkit/bdk#1178: `LocalChain` with hardwired genesis block

f1b112e8f9563c2afb9097911ee5e1afbbc55d22 docs(bitcoind_rpc): update docs for `Emitter::new` (志宇)
9a250baf6203a077fcfc03c3f6b386d5fba03d60 chore: make clippy happy (志宇)
79b84bed0ec399a375490c2c0f16c6ea3f5969b6 feat(bdk): changeset's `Append` impl checks that network is consistent (志宇)
06a956ad20ca5f1dcd108e45a1a1fed924128cdb feat!: change `load_from_persistence` to return an option (志宇)
c3265e2514070bd4da92ca343fe884e13e831360 test(bdk): add tests for wallet constructor methods (志宇)
96f1d94e2c8378d820a59e5ceafe686477243bf8 test(file_store): add construction method tests (志宇)
1886dc4fe743408132c3257afc1cacbb4d964105 chore(examples): use `Wallet::new_or_load` method where appropriate (志宇)
24994a3ed47aacf203ca0b456eb22f4b93ec58a8 feat(file_store)!: have separate methods for creating and opening Store (志宇)
d294e2e3189dc14efe5b9916cf83f5e7f8592c64 feat(wallet)!: add `new_or_load` methods (志宇)
7c6cbc4d9f7349e769594151936b3e2947b79d00 chore(file_store): rm empty test file (志宇)
6cf3963c6cfeacc8cd9d59bdb0bafded5022baaf feat(bdk)!: have separate methods for creating and loading `Wallet` (志宇)
7d5f31f6cc323241e866c136656f9674e0bf7c53 feat(chain, file_store): add `is_empty` method to `PersistBackend` trait (志宇)
5998a228191caccfaeac2b38ed4f319994b944c1 feat!: `LocalChain` with hardwired genesis checkpoint (志宇)

Pull request description:

  closes #1079
  closes #1107

  ### Description

  Many methods of `TxGraph` require a `chain_tip: BlockId` input to use against a `ChainOracle` implementation. This is used to ask the `ChainOracle` implementation whether a certain block exists in the chain identified by the `chain_tip`. This guarantees that the `TxGraph` methods will return a consistent history of transactions.

  However, the `ChainOracle` trait's `get_chain_tip` method returns an option of `BlockId`. It becomes unclear what to do when `get_chain_tip` returns `None`.

  This PR changes the `ChainOracle::get_chain_tip` method to always return a `BlockId` (no `Option`). `LocalChain` now hardwires the genesis block in order to implement `ChainOracle`.

  `bdk::Wallet` and `bdk_file_store::Store` are changed to have separate constructor methods for initializing a fresh instance and recovering a previous instance from persistence.

  ### Notes to the reviewers

  ### Changelog notice

  - Changed `ChainOracle::get_chain_tip` method to return a `BlockId` instead of an `Option` of a `BlockId`.
  - Refactored `LocalChain` so that the genesis `BlockId` is hardwired. This way, the `ChainOracle::get_chain_tip` implementation can always return a tip.
  - Add `is_empty` method to `PersistBackend`. This returns true when there is no data in the persistence.
  - Changed `Wallet::new` to initialize a fresh wallet only.
  - Added `Wallet::load` to restore an instance of a wallet.
  - Replaced `Store::new` with separate methods to create/open the database file.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

Top commit has no ACKs.

Tree-SHA512: 31b75fb53cc451f1fce7e409f1112c43973db7e8b5b31640e01e5b52089683b60320565427d6ea0478ff4c8680dbdb9272fdab08aef69d30f257da52e731e1a3