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

.github/workflows/cont_integration.yml

@@ -16,14 +16,16 @@ jobs:
           - default
           - minimal
           - all-keys
-          - minimal,esplora
+          - minimal,esplora,ureq
           - key-value-db
           - electrum
           - compact_filters
-          - esplora,key-value-db,electrum
+          - esplora,ureq,key-value-db,electrum
           - compiler
           - rpc
           - verify
+          - async-interface
+          - async-interface,esplora,reqwest
     steps:
       - name: checkout
         uses: actions/checkout@v2
@@ -137,7 +139,8 @@ jobs:
       - name: Update toolchain
         run: rustup update
       - name: Check
-        run: cargo check --target wasm32-unknown-unknown --features esplora --no-default-features
+        run: cargo check --target wasm32-unknown-unknown --features esplora,reqwest --no-default-features
+
 
   fmt:
     name: Rust fmt