* Improve Runtime execution by caching runtime lookup
Cache whether the native or wasm runtime should be used for a given
code and if the latter, keep around the parsed wasmi::Module for faster
execution.
* Additional comment and code style fixes
* Fallback to WASM runtime if we can't call the version function
* The previous assumption that the wasm-code was compiled with rustc might be wrong now, that the code comes from the blockchain. Added Notes about that.