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/test/util_tests.cpp

@@ -1224,7 +1224,7 @@ BOOST_AUTO_TEST_CASE(test_ToLower)
     BOOST_CHECK_EQUAL(ToLower('Z'), 'z');
     BOOST_CHECK_EQUAL(ToLower('['), '[');
     BOOST_CHECK_EQUAL(ToLower(0), 0);
-    BOOST_CHECK_EQUAL(ToLower(255), 255);
+    BOOST_CHECK_EQUAL(ToLower('\xff'), '\xff');
 
     std::string testVector;
     Downcase(testVector);
@@ -1246,7 +1246,7 @@ BOOST_AUTO_TEST_CASE(test_ToUpper)
     BOOST_CHECK_EQUAL(ToUpper('z'), 'Z');
     BOOST_CHECK_EQUAL(ToUpper('{'), '{');
     BOOST_CHECK_EQUAL(ToUpper(0), 0);
-    BOOST_CHECK_EQUAL(ToUpper(255), 255);
+    BOOST_CHECK_EQUAL(ToUpper('\xff'), '\xff');
 }
 
 BOOST_AUTO_TEST_CASE(test_Capitalize)

src/uint256.cpp

@@ -33,7 +33,7 @@ void base_blob<BITS>::SetHex(const char* psz)
         psz++;
 
     // skip 0x
-    if (psz[0] == '0' && ToLower((unsigned char)psz[1]) == 'x')
+    if (psz[0] == '0' && ToLower(psz[1]) == 'x')
         psz += 2;
 
     // hex string to uint

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)

src/util/strencodings.h

@@ -208,7 +208,7 @@ NODISCARD bool ParseHDKeypath(const std::string& keypath_str, std::vector<uint32
  * @return          the lowercase equivalent of c; or the argument
  *                  if no conversion is possible.
  */
-constexpr unsigned char ToLower(unsigned char c)
+constexpr char ToLower(char c)
 {
     return (c >= 'A' && c <= 'Z' ? (c - 'A') + 'a' : c);
 }
@@ -229,7 +229,7 @@ void Downcase(std::string& str);
  * @return          the uppercase equivalent of c; or the argument
  *                  if no conversion is possible.
  */
-constexpr unsigned char ToUpper(unsigned char c)
+constexpr char ToUpper(char c)
 {
     return (c >= 'a' && c <= 'z' ? (c - 'a') + 'A' : c);
 }