refactor: reduce memory consumption of CachedSource

[h-a-n-a]

Use Rope to keep only a single copy of source. Previously, source was stored
for multiple times as long as cache was used.

Instead of creating a continuous string, Rope stores slices of Strings
and works like a string with a few string-like methods provided.

Known penalty:
Slicing Rope would likely to be slower if a large amount of chunks was stored.
Each byte_slice method would cost O(logN) time complexity to find the slice-containing chunks.

[codspeed-hq]

CodSpeed Performance Report

Merging #144 will not alter performance

_{Comparing refactor-cache (dea438d) with main (d85db3f)}

Summary

✅ 7 untouched benchmarks