Add a ureq version of esplora module

The `Blockchain` implementation for connecting to an Esplora instance is
currently based on `reqwest`. Some users may not wish to use reqwest.

`ureq` is a simple HTTP client (no async) that is useful when `reqwest`
is not suitable.

- Move `esplora.rs` -> `esplora/reqwest.rs`
- Add an implementation based on the `reqwest` esplora code but using `ureq`
- Add feature flags and conditional includes to re-export everything to
  the `esplora` module so we don't effect the rest of the code base.
- Remove the forced dependency on `tokio`.
- Make esplora independent of async-interface
- Depend on local version of macros crate

src/blockchain/any.rs

@@ -37,9 +37,9 @@
 //! )?;
 //! # }
 //!
-//! # #[cfg(feature = "esplora")]
+//! # #[cfg(all(feature = "esplora", feature = "ureq"))]
 //! # {
-//! let esplora_blockchain = EsploraBlockchain::new("...", None, 20);
+//! let esplora_blockchain = EsploraBlockchain::new("...", 20);
 //! let wallet_esplora: Wallet<AnyBlockchain, _> = Wallet::new(
 //!     "...",
 //!     None,
@@ -60,6 +60,8 @@
 //! # use bdk::blockchain::*;
 //! # use bdk::database::MemoryDatabase;
 //! # use bdk::Wallet;
+//! # #[cfg(all(feature = "esplora", feature = "ureq"))]
+//! # {
 //! let config = serde_json::from_str("...")?;
 //! let blockchain = AnyBlockchain::from_config(&config)?;
 //! let wallet = Wallet::new(
@@ -69,6 +71,7 @@
 //!     MemoryDatabase::default(),
 //!     blockchain,
 //! )?;
+//! # }
 //! # Ok::<(), bdk::Error>(())
 //! ```