util: Make ToLower and ToUpper take a char

Unfortunately, `std::string` elements are (bare) chars. As these
are the most likely type to be passed to these functions, make them use
char instead of unsigned char. This avoids some casts.

src/util/strencodings.cpp

@@ -589,7 +589,7 @@ bool ParseHDKeypath(const std::string& keypath_str, std::vector<uint32_t>& keypa
 
 void Downcase(std::string& str)
 {
-    std::transform(str.begin(), str.end(), str.begin(), [](unsigned char c){return ToLower(c);});
+    std::transform(str.begin(), str.end(), str.begin(), [](char c){return ToLower(c);});
 }
 
 std::string Capitalize(std::string str)