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/esplora/mod.rs

@@ -0,0 +1,124 @@
+//! Esplora
+//!
+//! This module defines a [`EsploraBlockchain`] struct that can query an Esplora
+//! backend populate the wallet's [database](crate::database::Database) by:
+//!
+//! ## Example
+//!
+//! ```no_run
+//! # use bdk::blockchain::esplora::EsploraBlockchain;
+//! let blockchain = EsploraBlockchain::new("https://blockstream.info/testnet/api", 20);
+//! # Ok::<(), bdk::Error>(())
+//! ```
+//!
+//! Esplora blockchain can use either `ureq` or `reqwest` for the HTTP client
+//! depending on your needs (blocking or async respectively).
+//!
+//! Please note, to configure the Esplora HTTP client correctly use one of:
+//! Blocking:  --features='esplora,ureq'
+//! Async:     --features='async-interface,esplora,reqwest' --no-default-features
+use std::fmt;
+use std::io;
+
+use serde::Deserialize;
+
+use bitcoin::consensus;
+use bitcoin::{BlockHash, Txid};
+
+#[cfg(all(
+    feature = "esplora",
+    feature = "reqwest",
+    any(feature = "async-interface", target_arch = "wasm32"),
+))]
+mod reqwest;
+
+#[cfg(all(
+    feature = "esplora",
+    feature = "reqwest",
+    any(feature = "async-interface", target_arch = "wasm32"),
+))]
+pub use self::reqwest::*;
+
+#[cfg(all(
+    feature = "esplora",
+    not(any(
+        feature = "async-interface",
+        feature = "reqwest",
+        target_arch = "wasm32"
+    )),
+))]
+mod ureq;
+
+#[cfg(all(
+    feature = "esplora",
+    not(any(
+        feature = "async-interface",
+        feature = "reqwest",
+        target_arch = "wasm32"
+    )),
+))]
+pub use self::ureq::*;
+
+/// Data type used when fetching transaction history from Esplora.
+#[derive(Deserialize)]
+pub struct EsploraGetHistory {
+    txid: Txid,
+    status: EsploraGetHistoryStatus,
+}
+
+#[derive(Deserialize)]
+struct EsploraGetHistoryStatus {
+    block_height: Option<usize>,
+}
+
+/// Errors that can happen during a sync with [`EsploraBlockchain`]
+#[derive(Debug)]
+pub enum EsploraError {
+    /// Error during ureq HTTP request
+    #[cfg(feature = "ureq")]
+    Ureq(::ureq::Error),
+    /// Transport error during the ureq HTTP call
+    #[cfg(feature = "ureq")]
+    UreqTransport(::ureq::Transport),
+    /// Error during reqwest HTTP request
+    #[cfg(feature = "reqwest")]
+    Reqwest(::reqwest::Error),
+    /// HTTP response error
+    HttpResponse(u16),
+    /// IO error during ureq response read
+    Io(io::Error),
+    /// No header found in ureq response
+    NoHeader,
+    /// Invalid number returned
+    Parsing(std::num::ParseIntError),
+    /// Invalid Bitcoin data returned
+    BitcoinEncoding(bitcoin::consensus::encode::Error),
+    /// Invalid Hex data returned
+    Hex(bitcoin::hashes::hex::Error),
+
+    /// Transaction not found
+    TransactionNotFound(Txid),
+    /// Header height not found
+    HeaderHeightNotFound(u32),
+    /// Header hash not found
+    HeaderHashNotFound(BlockHash),
+}
+
+impl fmt::Display for EsploraError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self)
+    }
+}
+
+impl std::error::Error for EsploraError {}
+
+#[cfg(feature = "ureq")]
+impl_error!(::ureq::Error, Ureq, EsploraError);
+#[cfg(feature = "ureq")]
+impl_error!(::ureq::Transport, UreqTransport, EsploraError);
+#[cfg(feature = "reqwest")]
+impl_error!(::reqwest::Error, Reqwest, EsploraError);
+impl_error!(io::Error, Io, EsploraError);
+impl_error!(std::num::ParseIntError, Parsing, EsploraError);
+impl_error!(consensus::encode::Error, BitcoinEncoding, EsploraError);
+impl_error!(bitcoin::hashes::hex::Error, Hex, EsploraError);