Split ext_clear_storage out from ext_set_storage (#5103)

* split out ext_clear_storage from ext_set_storage contracts API

* update tests to adjust for the ext_set_storage changes

* adjust COMPLEXITY for the ext_set_storage API changes

* remove value_len == 0 constraint for ext_set_storage

* bump spec_version

* remove guarantee from COMPLEXITY of ext_clear_storage

Co-authored-by: Gavin Wood <gavin@parity.io>