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/lib.rs

@@ -205,11 +205,27 @@ extern crate serde;
 #[macro_use]
 extern crate serde_json;
 
+#[cfg(all(feature = "reqwest", feature = "ureq"))]
+compile_error!("Features reqwest and ureq are mutually exclusive and cannot be enabled together");
+
 #[cfg(all(feature = "async-interface", feature = "electrum"))]
 compile_error!(
     "Features async-interface and electrum are mutually exclusive and cannot be enabled together"
 );
 
+#[cfg(all(feature = "async-interface", feature = "ureq"))]
+compile_error!(
+    "Features async-interface and ureq are mutually exclusive and cannot be enabled together"
+);
+
+#[cfg(all(feature = "async-interface", feature = "compact_filters"))]
+compile_error!(
+    "Features async-interface and compact_filters are mutually exclusive and cannot be enabled together"
+);
+
+#[cfg(all(feature = "esplora", not(feature = "ureq"), not(feature = "reqwest")))]
+compile_error!("Feature missing: esplora requires either ureq or reqwest to be enabled");
+
 #[cfg(feature = "keys-bip39")]
 extern crate bip39;
 
@@ -228,9 +244,6 @@ pub extern crate bitcoincore_rpc;
 #[cfg(feature = "electrum")]
 pub extern crate electrum_client;
 
-#[cfg(feature = "esplora")]
-pub extern crate reqwest;
-
 #[cfg(feature = "key-value-db")]
 pub extern crate sled;