* Flattens TransactionContext::instruction_trace.
* Stop the search at transaction level.
* Renames get_instruction_context_at => get_instruction_context_at_nesting_level.
* Removes TransactionContext::get_instruction_trace().
Adds TransactionContext::get_instruction_trace_length() and TransactionContext::get_instruction_context_at_index().
* Have TransactionContext::instruction_accounts_lamport_sum() accept an iterator instead of a slice.
* Removes instruction_trace from ExecutionRecord.
* make InstructionContext::new() private
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