Support Wasm sign extension and bulk memory extension

Take a look into casper-wasmi/casper-wasm and asses possibility to support sign extension and bulk memory extension without dealing with wasmi upgrade. If it’s possible by enabling flags, we should do it, otherwise we’d have to wait and upgrade wasmi first (ticket above). Modern compilers i.e. rust, and golang are emitting these extensions by default, so if we want to support these compilers we should also support these extensions.