Prefer the term "instruction processor" over "program". Reserve
the term "native" for the loader and shared object it loads.
Compiling an instruction processor to BPF shouldn't imply changing
to a non-native entrypoint.
* 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
Rob Walker
Michael Vines
Greg Fitzgerald