std::unordered_map is better than std::map

Since the key logic is that given the plain string, find the respond unicode string, the unordered_map is better.

I did some simple test case for both, below are the test result:

std::unordered_map version:
Elapsed time: 0.000145379 s
Elapsed time: 0.000127852 s
Elapsed time: 0.00012735 s
Elapsed time: 0.00012398 s
Elapsed time: 0.000123349 s
Elapsed time: 0.000126856 s

std::map version:
Elapsed time: 0.000279197 s
Elapsed time: 0.000269347 s
Elapsed time: 0.000259659 s
Elapsed time: 0.000255836 s
Elapsed time: 0.000258944 s
Elapsed time: 0.000256731 s

No source file or test is named in the issue. Start by locating the string-to-Unicode mapping implementation and checking whether it relies on std::map ordering; compare the existing behavior with the reported unordered_map benchmark. Done means preserving the lookup results while confirming the change with the project's available tests or benchmark.

Written by the indexing model from the issue text.