refactor: replace sizeof(a)/sizeof(a[0]) by std::size (C++17)

Removes the macro ARRAYLEN and also substitutes all other uses of the same
"sizeof(a)/sizeof(a[0])" pattern by std::size, available since C++17.

src/util/strencodings.h

@@ -17,8 +17,6 @@
 #include <string>
 #include <vector>
 
-#define ARRAYLEN(array)     (sizeof(array)/sizeof((array)[0]))
-
 /** Used by SanitizeString() */
 enum SafeChars
 {