Term userdata is confusing

[garious]

Problem

Solana's use of the term userdata comes from Lua and BREW, and isn't too meaningful without that historical context. Also, we use userdata to mean [at least] two different things. We use it to pass "arguments" to instructions, and we use it to store arbitrary data in program accounts. In both cases, the userdata can only be interpreted by a single program, but the error produced is ambiguous InvalidUserdata. They ought to have different, more precise names and unique error codes.

Proposed Solution

• Rename Instruction::userdata to Instruction::argdata
• Rename Account::userdata to Account::data
• Split ProgramError::InvalidUserdata into InvalidArgdata and InvalidData. It shouldn't be possible to ever see InvalidData; that'd be a program bug, not a bad transaction.

[garious]

Update: instead of saying "there's argdata in instructions and data in accounts", I'd prefer to say, "there's instruction data and account data".

• Rename Instruction::userdata to Instruction::data
• Rename Account::userdata to Account::data
• Split ProgramError::InvalidUserdata into InvalidInstructionData and InvalidAccountData. It shouldn't be possible to ever see InvalidAccountData; that'd be a program bug, not a bad transaction.