Add benchmarks for Quantity operations

[AkramBitar]

Summary

• Add benchmarks for BigQuantity and UInt64Quantity operations (Add, Sub, Cmp)
• Add benchmark for ToQuantity parsing

Continues the work from #1477 (closed), rebased on latest main with review comments addressed:

• Assert on errors instead of ignoring them (require.NoError)
• Move setup out of the timed loop and call b.ResetTimer() so only the target op is measured
• Store side-effect-free results in package-level sinks to prevent dead-code elimination

Resolves #1819