Performance improvements

[cokicm]

Description

Multiple performance improvements:
- Pooling of big.Int instances on the stack to avoid frequent allocations/cleanup
- Removing unnecessary big.Int allocation in pop() method and fixing tests
- Avoiding unnecessary op.String() calls when tracer is not initialized.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Since there is not yet official performance framework, I have tested this fix with custom code.