Expose a lower-level API for encoding C string.

Add json_encode_string() as another API for directly encode
C string into JSON string.

parson.c

@@ -924,6 +924,22 @@ static int json_serialize_string(const char *string, char *buf) {
     return written_total;
 }
 
+char *json_encode_string(const char *string) {
+    char *enc_str = NULL;
+    int buf_size_bytes = json_serialize_string(string, NULL);
+
+    if (buf_size_bytes <= 0) {
+        return NULL;
+    }
+
+    enc_str = (char*)malloc(buf_size_bytes + 1);
+    if (enc_str == NULL)
+        return NULL;
+
+    json_serialize_string(string, enc_str);
+    return enc_str;
+}
+
 static int append_indent(char *buf, int level) {
     int i;
     int written = -1, written_total = 0;

parson.h

@@ -75,6 +75,7 @@ JSON_Value * json_parse_string(const char *string);
 JSON_Value * json_parse_string_with_comments(const char *string);
 
 /* Serialization */
+char *      json_encode_string(const char *string);
 size_t      json_serialization_size(const JSON_Value *value); /* returns 0 on fail */
 JSON_Status json_serialize_to_buffer(const JSON_Value *value, char *buf, size_t buf_size_in_bytes);
 JSON_Status json_serialize_to_file(const JSON_Value *value, const char *filename);

tests.c

@@ -48,6 +48,7 @@ void test_suite_6(void); /* Test value comparing verification */
 void test_suite_7(void); /* Test schema validation */
 void test_suite_8(void); /* Test serialization */
 void test_suite_9(void); /* Test serialization (pretty) */
+void test_suite_10(void); /* Test string encoding */
 
 void print_commits_info(const char *username, const char *repo);
 void persistence_example(void);
@@ -74,6 +75,7 @@ int main() {
     test_suite_7();
     test_suite_8();
     test_suite_9();
+    test_suite_10();
     printf("Tests failed: %d\n", tests_failed);
     printf("Tests passed: %d\n", tests_passed);
     return 0;
@@ -465,6 +467,14 @@ void test_suite_9(void) {
     TEST(STREQ(file_contents, serialized));
 }
 
+void test_suite_10(void) {
+    const char *raw_str = "\"\\/\b\f\n\r\t\"test";
+    char *enc_str = json_encode_string(raw_str);
+
+    TEST(STREQ(enc_str, "\"\\\"\\\\\\/\\b\\f\\n\\r\\t\\\"test\""));
+    free(enc_str);
+}
+
 void print_commits_info(const char *username, const char *repo) {
     JSON_Value *root_value;
     JSON_Array *commits;