tests/test-gendoc-zh_TW.t

rust: simply use TryInto to convert slice to array

Since our rust module depends on TryInto, there's no point to avoid using it.

While rewriting copy_into_array(), I noticed CPython interface doesn't check
the length of the p1/p2 values, which is marked as TODO.

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors