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

main-unix.c

@@ -560,8 +560,8 @@ main (int argc,
 
       jerry_api_size_t err_str_size = jerry_api_get_string_size (err_str_p);
       JERRY_ASSERT (err_str_size < 256);
-      ssize_t sz = jerry_api_string_to_char_buffer (err_str_p, err_str_buf, err_str_size);
-      JERRY_ASSERT (sz > 0);
+      jerry_api_size_t sz = jerry_api_string_to_char_buffer (err_str_p, err_str_buf, err_str_size);
+      JERRY_ASSERT (sz == err_str_size);
       err_str_buf[err_str_size] = 0;
 
       JERRY_ERROR_MSG ("%s\n", err_str_buf);