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
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