Improve Runtime execution by caching runtime lookup (#276)

* 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.