Add jerry_string_to_utf8_char_buffer API function. (#1501)

Copy the characters of a string into a specified utf-8 string buffer.

JerryScript-DCO-1.0-Signed-off-by: Robert Sipka rsipka.uszeged@partner.samsung.com

docs/02.API-REFERENCE.md

@@ -1352,6 +1352,49 @@ jerry_string_to_char_buffer (const jerry_value_t value,
 - [jerry_create_string](#jerry_create_string)
 - [jerry_get_string_size](#jerry_get_string_size)
 
+## jerry_string_to_utf8_char_buffer
+
+**Summary**
+
+Copy the characters of a string into a specified utf-8 buffer.
+The '\0' character could occur in character buffer. Returns 0,
+if the value parameter is not a string or the buffer isn't
+large enough for the whole string.
+
+**Prototype**
+
+```c
+jerry_size_t
+jerry_string_to_utf8_char_buffer (const jerry_value_t value,
+                                  jerry_char_t *buffer_p,
+                                  jerry_size_t buffer_size);
+```
+
+- `value` - input string value
+- `buffer_p` - pointer to output buffer
+- `buffer_size` - size of the buffer
+- return value - number of bytes, actually copied to the buffer
+
+**Example**
+
+```c
+{
+  jerry_value_t value;
+  ... // create or acquire value
+
+  jerry_size_t req_sz = jerry_get_utf8_string_size (value);
+  jerry_char_t str_buf_p[req_sz];
+
+  jerry_string_to_utf8_char_buffer (value, str_buf_p, req_sz);
+
+  jerry_release_value (value);
+}
+```
+
+**See also**
+
+- [jerry_create_string_from_utf8](#jerry_create_string_from_utf8)
+- [jerry_get_utf8_string_size](#jerry_get_utf8_string_size)
 
 # Functions for array object values