unify native and WASM VP host env interfaces

[tzemanovic]

We'll be adding a Ctx arg to the wasm execution on which we can implement the same trait as for the native VP - then any library code can build on top of the trait for re-usability. (This is better than the alternative, in which we would keep the same interface in WASM and somehow "hack" native VP to use implicit Ctx)

Although not strictly necessary, we should also apply the same change to transaction host env to keep them consistent.