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

.github/workflows/rust.yml

@@ -58,11 +58,21 @@ jobs:
             - name: Cargo check all targets.
               run: cargo check --all-targets
 
-            # Next, check each feature on its own and compile each crate separately. This is no good
-            # for subxt/examples, which expect default features to be enabled, hence the above check
-            # and why we don'#t do `--all-targets` for this one.
+            # Next, check subxt features.
+            # - `native` feature must always be enabled
+            # - `web` feature is always ignored.
+            # - This means, don't check --no-default-features and don't try enabling --all-features; both will fail
+            - name: Cargo hack; check each subxt feature
+              run: cargo hack -p subxt --each-feature check --exclude-no-default-features --exclude-all-features --exclude-features web --features native
+
+            # Subxt-signer has the "subxt" features enabled in the "check all targets" test. Run it on its own to
+            # check it without. We can't enable subxt or web features here, so no cargo hack.
+            - name: Cargo check subxt-signer
+              run: cargo check -p subxt-signer
+
+            # Next, check each other package in isolation.
             - name: Cargo hack; check each feature/crate on its own
-              run: cargo hack --exclude-all-features --each-feature check --workspace
+              run: cargo hack --exclude subxt --exclude subxt-signer --exclude-all-features --each-feature check --workspace
 
     fmt:
         name: Cargo fmt
@@ -215,10 +225,16 @@ jobs:
                   mkdir -p ~/.local/bin
                   mv substrate ~/.local/bin
 
-            - name: Run WASM tests
+            - name: Run subxt WASM tests
               run: |
                   substrate --dev --tmp > /dev/null 2>&1 &
                   wasm-pack test --headless --firefox
                   wasm-pack test --headless --chrome
                   pkill substrate
               working-directory: testing/wasm-tests
+
+            - name: Run subxt-signer WASM tests
+              run: |
+                  wasm-pack test --headless --firefox
+                  wasm-pack test --headless --chrome
+              working-directory: signer/wasm-tests