Use binarysearch in Level Compaction and refactor function compact

[ppdogg]

Disscussed in issue#136

[skyzh]

The binary-search overlap calculation has blocking edge cases. If the requested key range falls in a gap between two SSTs, lower can be greater than upper, so the inclusive slice panics. If the range lies before the first SST, saturating_sub(1) makes upper equal to zero and incorrectly selects the first SST even though there is no overlap.

Please keep the upper bound exclusive, return an empty result when lower is greater than or equal to upper_exclusive, and slice with lower..upper_exclusive. This also needs tests for ranges before the first table, between tables, and after the last table. The unrelated compact refactor would be easier to assess separately.

🤖 (posted by Codex)