Update the genToc funtion to make the name required in the Code node,
and add an additional optional field for the expected error, to use with
test_err, test_safety and obj_err.

Update langref.html.in to ensure all code blocks have a name that

  - is unique, so that a doctest can be identified by it
  - is descriptive

For test, test_err and test_safefy, ensure that the doctest name starts
with "test_", excluding doctests in the "Zig Test" section and doctests
that are imported by other doctests.

Ensure that the indentation of code_begin and code_end blocks are
consistent.

Fix a typo in pointer_arthemtic.