Added implementations of strchr, strcspn and strspn

tests/string/strchr.c

@@ -0,0 +1,10 @@
+#include <string.h>
+#include <stdio.h>
+
+int main(int argc, char* argv[]) {
+	printf("%s\n", strchr("hello", 'e')); // should be ello
+	printf("%s\n", strchr("world", 'l')); // should be ld
+	printf("%s\n", strchr("world", 0)); // should be ''
+
+    return 0;
+}

tests/string/strcspn.c

@@ -0,0 +1,9 @@
+#include <string.h>
+#include <stdio.h>
+
+int main(int argc, char* argv[]) {
+	printf("%ld\n", strcspn("hello", "world")); // should be 2
+	printf("%ld\n", strcspn("banana", "world")); // should be 6
+
+    return 0;
+}

tests/string/strspn.c

@@ -0,0 +1,10 @@
+#include <string.h>
+#include <stdio.h>
+
+int main(int argc, char* argv[]) {
+	printf("%lu\n", strspn("hello", "hello")); // should be 5
+	printf("%lu\n", strspn("world", "wendy")); // should be 1
+	printf("%lu\n", strspn("banana", "apple")); // should be 0
+
+    return 0;
+}