lib: c: core: implement strchr and strcspn

lib/libc/core/string/strcspn.c

@@ -0,0 +1,15 @@
+#include <stddef.h>
+
+size_t strcspn(const char *dest, const char *src)
+{
+	size_t i = 0;
+	for (i = 0; dest[i]; i++) {
+		for (size_t ii = 0; src[ii]; i++) {
+			if (dest[i] == src[ii]) {
+				return i;
+			}
+		}
+	}
+
+	return i;
+}