Leave rust-bitcoin compilation flags untouched

Because `wasm_bindgen` enables a hobbled `std` mode by default
(where you can "use" the full std, but random bits of it panic),
`wasm_bindgen` users really should use `no_std` builds where
possible. However, cargo makes this very difficult - crates with
`no_std` flags can be silently made `std` just by one dependency
setting the flag silently.

This happens here by leaving default-features enabled. Luckily, we
can simply disable `default-features` and everything else remains.

Author: TheBlueMatt

Cargo.toml

@@ -17,7 +17,7 @@ path = "src/lib.rs"
 
 [dependencies]
 serde = { version = "1.0", features = ["derive"] }
-bitcoin = { version = "0.29.1", features = ["serde"] }
+bitcoin = { version = "0.29.1", features = ["serde"], default-features = false }
 log = "^0.4"
 ureq = { version = "2.5.0", features = ["json"], optional = true }
 reqwest = { version = "0.11", optional = true, default-features = false, features = ["json"] }