Fixes issue where getAddress and sendTransaction could not be found for MockSigner in tests. The problem was that the async: raises update had not been applied to the MockSigner.
- removes async: raises from getAddress and signTransaction because derived JsonRpcSigner methods were not being used when dynamically dispatched. Once `raises` was removed from the async annotation, the dynamic dispatch worked again. This is only the case for getAddress and signTransaction.
- add gcsafe annotation to wallet.provider so that it matches the base method
- signer procs raise SignerError, provider procs raise ProviderError
- WalletError now inherits from SignerError
- move wallet module under signers
- create jsonrpo moudle under signers
- bump nim-json-rpc for null-handling fixes
- All jsonrpc provider tests passing, still need to fix others
