* `light::verify_transaction` basic tx validation
* update wasm tests
* Provide `cached_nonce` in `parity_next_nonce` RPC
* nits
* Improve error handeling
Two separate errors for distinguishing between `account not found` and
`insufficient balance`. However, when `next_nonce()` is called and the
account is not found then provide `local_best_next_nonce`!
* Ensure only one n/w request is performed
Refactored to code again:
* Removed `fn cached_next_nonce`
* Removed extra n/w request in `sign` to check balance
* Refactored `fill_optional_field` to request nonce and check account balance
* nits
* grumbles needless clone
* Prevent integer overflow with saturating add & mul
* Call `sign_transaction()` directly from `sign()`
Because the change in `fill_optional_fields` always fill the nonce it is
now possible to call `sign_transaction` directly instead of creating a
`ProspectiveSigner` "object".
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