Add subxt_signer crate for native & WASM compatible signing (#1016)

* Add and use subxt-signer crate for WASM compatible signing

* cargo fmt

* dev keypairs already references

* WIP fix various breakages

* re-jig features to be simpler and various test fixes etc

* doc and web fix

* fix various bits and pieces

* fix a test I broke

* dev-deps can't be linked to in docs, hrmph

* cargo fmt

* another doc link

* document the subxt_signer crate more thoroughly

* move feature flag for consistency

* more docs, no default subxt feature flag on signer, update release instrs

* Add missing license header

* unwrap_inner => into_inner

* extend a test a little to better check derive junctions

* note more clearly that the crypto bits come from sp_core::crypto

subxt/src/lib.rs

@@ -33,23 +33,31 @@
 )]
 #![allow(clippy::type_complexity)]
 
+#[cfg(any(
+    all(feature = "web", feature = "native"),
+    not(any(feature = "web", feature = "native"))
+))]
+compile_error!("subxt: exactly one of the 'web' and 'native' features should be used.");
+
+#[cfg(all(feature = "web", feature = "substrate-compat"))]
+compile_error!("subxt: the 'substrate-compat' feature is not compatible with the 'web' feature.");
+
 // The guide is here.
 pub mod book;
 
 // Suppress an unused dependency warning because tokio is
 // only used in example code snippets at the time of writing.
 #[cfg(test)]
-use tokio as _;
+mod only_used_in_docs_or_tests {
+    use subxt_signer as _;
+    use tokio as _;
+}
 
 // Used to enable the js feature for wasm.
-#[cfg(target_arch = "wasm32")]
+#[cfg(feature = "web")]
+#[allow(unused_imports)]
 pub use getrandom as _;
 
-#[cfg(all(feature = "jsonrpsee-ws", feature = "jsonrpsee-web"))]
-std::compile_error!(
-    "Both the features `jsonrpsee-ws` and `jsonrpsee-web` are enabled which are mutually exclusive"
-);
-
 pub mod blocks;
 pub mod client;
 pub mod config;