Cache ScriptHash

[shargon]

With this PR we could improve the performance of the VM.

Before the change:

• The VM has the hash cached per ExecutionContext, so every CALL or new ExecutionContext if you want to know the ScriptHash (very usual), you need to compute again the script hash.

After the change:

• The script hash is computed only once for the script, is cached even though we destroy the ExecutionContext

[vncoelho]

Sounds good, @shargon

[shargon]

Maybe ICrypto should be static inside the VM, what do you think?

[shargon]

@erikzhang nice improve :)