Merge bitcoindevkit/bdk#756: Remove genesis_block lazy initialization

e6f2d029fa9708f98599c1bd4ef74d232b111c5a Remove genesis_block lazy initialization (Shobit Beltangdy)

Pull request description:

  ### Description

  This commit contains a change to address issue #752

  cargo test runs successfully.

  ### Notes to the reviewers

  Hi, newbie here learning Rust and BDK!  I've removed the lazy_static block in this commit, and when learning about lazy_static also came across something called [once_cell](https://doc.rust-lang.org/std/cell/struct.OnceCell.html), [soon to be available in stdlib](https://github.com/rust-lang/rust/issues/74465).  It's like lazy_static but faster and is not a macro.  Shall I keep the lazy_static and create an issue to switch to once_cell in the future, or remove the lazy_static (as I've done) and create an issue anyway?

  #### 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

ACKs for top commit:
  notmandatory:
    ACK e6f2d029fa9708f98599c1bd4ef74d232b111c5a

Tree-SHA512: 528f5fdfb0d7d1f7a83869b7a0de1b25dfcfafae2671c9229cdb4e5d80d11e5578d9325b3d95555e59f5dfb4ed6304c0c112a01a56b6a596e50dd62e310c516e

diff --cc Cargo.toml
Simple merge