Prevent account storage leakage (#270)

* WIP

* Iteration over all keys with the specified prefix

* Add clear_prefix in runtime-io

* Introduce a custom storage impl: Double Map

* Remove prefix

* Impl for_keys_with_prefix for light client

* Fix wasm_executor

* Test storage removal leads to removal of stroage

* Check for ok result in storage tests.

* Add docs.

* Remove commented code under decl_storage!

* Add clear_prefix test in runtime-io

* Add test for wasm_executor

* Prefix walking test.

* Rebuild binaries.