Drop ICU dependency on Win32 & fix min/max macros

[caesay]

As discussed in #145.

If _WIN32 and UNICODE are defined, it will use the win32 functions for ToLower/ToUpper instead of ICU.

Also, when including the Windows.h header, std::min and std::max conflict with the min/max windows macros, so I've wrapped them in parenthesis to fix this.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2b4f421) 96.66% compared to head (7e06484) 96.68%. Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #147 +/- ## ========================================== + Coverage 96.66% 96.68% +0.01% ========================================== Files 2 2 Lines 17417 17485 +68 ========================================== + Hits 16837 16905 +68 Misses 498 498 Partials 82 82 ```

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