Fix the stale cache problem

[j824h]

This is to fix the issue #151 .

A concern is that this will cause FCUseCache[] to actually cache the dependency indefinitely until FCClearCache[] is called manually. This might cause a new memory overhead when dependency is updated frequently, especially so since cachedToString[] does not employ MemSet[].

Test did run slightly slower. test-69324b9.txt test-ee27a8f.txt

Besides, I am curious if caching ToString[] is beneficial, since I would expect it is supposed to be merely a conversion.