str.h: Add coap_string_equal() and create coap_string_t as replacement for str

Rename struct str as struct coap_string_t and 'typedef coap_string_t str' for
backwards compatabilty with the intent of removing str at some point to
prevent potential name clashes when compiling application code.

Add in new coap_string_equal() function for consistent comparing of strings.

include/coap/str.h

@@ -12,22 +12,46 @@
 
 #include <string.h>
 
-typedef struct {
+typedef struct coap_string_t {
   size_t length;    /* length of string */
   unsigned char *s; /* string data */
-} str;
+} coap_string_t;
+
+/* For backwards compatability */
+typedef coap_string_t str;
 
 #define COAP_SET_STR(st,l,v) { (st)->length = (l), (st)->s = (v); }
 
 /**
- * Returns a new string object with at least size bytes storage allocated. The
- * string must be released using coap_delete_string();
+ * Returns a new string object with at least size+1 bytes storage allocated.
+ * The string must be released using coap_delete_string();
+ *
+ * @param size The size to allocate for the binary string data
+ *
+ * @return       A pointer to the new object or @c NULL on error.
  */
-str *coap_new_string(size_t size);
+coap_string_t *coap_new_string(size_t size);
 
 /**
  * Deletes the given string and releases any memory allocated.
+ *
+ * @param string The string to free off
  */
-void coap_delete_string(str *);
+void coap_delete_string(coap_string_t *string);
+
+/**
+ * Compares the two strings for equality
+ *
+ * @param string1 The first string
+ * @param string2 The second string
+ *
+ * @return         @c 1 if the strings are equal
+ *                 @c 0 otherwise.
+ */
+COAP_STATIC_INLINE int
+coap_string_equal(coap_string_t *string1, coap_string_t *string2) {
+  return string1->length == string2->length &&
+         memcmp (string1->s, string2->s, string1->length) == 0;
+}
 
 #endif /* _COAP_STR_H_ */