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

jerry-core/ecma/operations/ecma-eval.c

@@ -56,14 +56,13 @@ ecma_op_eval (ecma_string_t *code_p, /**< code string */
                             chars_num,
                             lit_utf8_byte_t);
 
-    const ssize_t buf_size = (ssize_t) chars_num;
-    ssize_t buffer_size_req = ecma_string_to_utf8_string (code_p,
-                                                          code_utf8_buffer_p,
-                                                          buf_size);
-    JERRY_ASSERT (buffer_size_req == buf_size);
+    lit_utf8_size_t buffer_size_req = ecma_string_to_utf8_string (code_p,
+                                                                  code_utf8_buffer_p,
+                                                                  chars_num);
+    JERRY_ASSERT (buffer_size_req == chars_num);
 
     ret_value = ecma_op_eval_chars_buffer ((jerry_api_char_t *) code_utf8_buffer_p,
-                                           (size_t) buf_size,
+                                           chars_num,
                                            is_direct,
                                            is_called_from_strict_mode_code);