port subxt-signer to no-std (such a pain)

signer/src/lib.rs

@@ -14,6 +14,9 @@
 //! subxt transactions for chains supporting sr25519 signatures.
 
 #![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+extern crate alloc;
 
 #[macro_use]
 mod utils;
@@ -40,9 +43,3 @@ pub use secrecy::{ExposeSecret, SecretString};
 // SecretUri's can be parsed from strings and used to generate key pairs.
 // DeriveJunctions are the "path" part of these SecretUris.
 pub use crypto::{DeriveJunction, SecretUri, SecretUriError, DEV_PHRASE};
-
-#[cfg(any(
-    all(feature = "web", feature = "native"),
-    not(any(feature = "web", feature = "native"))
-))]
-compile_error!("subxt-signer: exactly one of the 'web' and 'native' features should be used.");