levm: rethink transient storage

[JereSalo]

Transient Storage is currently an attribute of a callframe and I think that it should be part of the whole VM.
The idea of having a transient storage is to have a temporary storage that will be useful for the whole execution of a transaction, but then discarded. This is necessary when you want to share data between callframes in an easy way without using the actual storage, because that is expensive for something that you will end up discarding later.
EIP