add all missing wchar functions except floating point parsers

these are mostly untested and adapted directly from corresponding byte
string functions and similar.

src/string/wcsnlen.c

@@ -0,0 +1,8 @@
+#include <wchar.h>
+
+size_t wcsnlen(const wchar_t *s, size_t n)
+{
+	const wchar_t *z = wmemchr(s, 0, n);
+	if (z) n = z-s;
+	return n;
+}