verify_signature() was only used in a test that was testing
binary layout. It only worked because the test transaction only
had one signature.
from() was only used by verify_signature() and that's something
we'd typically called `pubkey()`.
hash() didn't return the hash of the Transaction, as you might
guess. It's only used for PoH, so move it into Entry.
* Rename userdata to data
Instead of saying "userdata", which is ambiguous and imprecise,
say "instruction data" or "account data".
Also, add `ProgramError::InvalidInstructionData`
Fixes#2761
Sathish Ambley
Greg Fitzgerald
Michael Vines