Fix problems arising from incorrect use of various size types

E.g.,
* `ssize_t` was used where `lit_utf8_size_t` or `jerry_api_size_t`
  would have been correct,
* `lit_utf8_size_t` was used where `ecma_length_t` would have been
  correct.

Note, the patch also includes internal and public API changes:
* `ecma_string_to_utf8_string` does not return negative value if
   output buffer is not large enough to contain the string; the
   buffer is expected to be large enough. (`ecma_string_get_size`
   can be used to retrieve the required size.)
* `jerry_api_string_to_char_buffer` adapts the same logic (and
  `jerry_api_get_string_size` can be used to determine the
  required size of the buffer).

Related issue: #942

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

docs/API-EXAMPLE.md

@@ -192,17 +192,14 @@ print_value (const jerry_api_value_t * value_p)
     // String value
     case JERRY_API_DATA_TYPE_STRING:
     {
-      ssize_t neg_req_sz, sz;
+      jerry_api_size_t req_sz, sz;
       // determining required buffer size
-      neg_req_sz = jerry_api_string_to_char_buffer (value_p->v_string,
-                                                    NULL,
-                                                    0);
-      assert (neg_req_sz < 0);
-      char * str_buf_p = (char*) malloc (-neg_req_sz);
+      req_sz = jerry_api_get_string_size (value_p->v_string);
+      char * str_buf_p = (char*) malloc (req_sz);
       sz = jerry_api_string_to_char_buffer (value_p->v_string,
                                             str_buf_p,
-                                            -neg_req_sz);
-      assert (sz == -neg_req_sz);
+                                            req_sz);
+      assert (sz == req_sz);
 
       printf ("%s", str_buf_p);