Make RuntypeBase more small shape

[yuhr]

The current interface RuntypeBase requires methods check, guard, and assert, but these APIs are built on validate. Probably it's better to have RuntypeBase only need validate method predefined and others are automatically populated internally, so that users can easily define their own runtypes, and make them integrated into runtypes' ecosystem.

Also we would provide a helper function to upgrade essential objects which have only validate method into full-fledged runtypes. (Maybe exposing create function?)