Implement set_top_level_instruction_index

[Arrowana]

Problem

anza made the hacky mollusk thing that doesn't work properly with instructions sysvar, because it is well, hacky.

The main issue is that it runs instruction one by one even if they belong to the same transaction with process_instructions_chain

anza-xyz/mollusk#156 The problem blocks this

Summary of Changes

Expose setting top_level_instruction_index on TransactionContext

Can we afford this smell?

[buffalojoec]

I think it's fine by me if we just put it under the dev-context-only-utils feature!

[Arrowana]

I think it's fine by me if we just put it under the dev-context-only-utils feature!

done

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.9%. Comparing base (1254878) to head (adc3f29).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #7845     +/-   ##
=========================================
- Coverage    82.9%    82.9%   -0.1%     
=========================================
  Files         823      823             
  Lines      360410   360413      +3     
=========================================
- Hits       299008   298976     -32     
- Misses      61402    61437     +35     

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