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/book/usage/runtime_apis.rs

@@ -22,8 +22,6 @@
 //! We can use the statically generated interface to build runtime calls:
 //!
 //! ```rust,no_run
-//! use sp_keyring::AccountKeyring;
-//!
 //! #[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata_small.scale")]
 //! pub mod polkadot {}
 //!
@@ -33,10 +31,8 @@
 //! Alternately, we can dynamically construct a runtime call:
 //!
 //! ```rust,no_run
-//! use sp_keyring::AccountKeyring;
 //! use subxt::dynamic::Value;
 //!
-//! let account = AccountKeyring::Alice.to_account_id();
 //! let runtime_call = subxt::dynamic::runtime_api_call(
 //!     "Metadata",
 //!     "metadata_versions",