* Introduces InstructionAccount which is like AccountMeta but uses an index instead of a Pubkey
* Renames InvokeContext::create_message() to InvokeContext::prepare_instruction()
* Removes Message and CompiledInstruction from InvokeContext interfaces.
* Resolves TODOs of sol_invoke_signed() in program-test.
* Moves CompiledInstruction::visit_each_account() into invoke_context.rs
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