util: refactor upper/lowercase functions

This includes renaming Downcase() to ToLower() and make it return a string rather than modify referenced arg.
Also adds ToUpper() string version.

src/util/strencodings.cpp

@@ -546,9 +546,18 @@ bool ParseFixedPoint(const std::string &val, int decimals, int64_t *amount_out)
     return true;
 }
 
-void Downcase(std::string& str)
+std::string ToLower(const std::string& str)
 {
-    std::transform(str.begin(), str.end(), str.begin(), [](char c){return ToLower(c);});
+    std::string r;
+    for (auto ch : str) r += ToLower((unsigned char)ch);
+    return r;
+}
+
+std::string ToUpper(const std::string& str)
+{
+    std::string r;
+    for (auto ch : str) r += ToUpper((unsigned char)ch);
+    return r;
 }
 
 std::string Capitalize(std::string str)