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

RELEASING.md

@@ -65,7 +65,6 @@ We also assume that ongoing work done is being merged directly to the `master` b
     2.  Perform a final sanity check that everything looks ok.
 
         ```
-        cargo hack --exclude-all-features --each-feature check --all-targets --workspace
         cargo test --all-targets
         ```
 
@@ -77,12 +76,10 @@ We also assume that ongoing work done is being merged directly to the `master` b
             (cd codegen && cargo publish) && \
             (cd macro && cargo publish) && \
             (cd subxt && cargo publish) && \
+            (cd signer && cargo publish) && \
             (cd cli && cargo publish);
         ```
 
-        If you run into any issues regarding crates not being able to find suitable versions of other `subxt-*` crates,
-        you may just need to wait a little longer and then run the remaining portion of that command.
-
 10. If the release was successful, tag the commit that we released in the `master` branch with the
     version that we just released, for example: