Set the "is_target" attribute for names, attributes and temps (anything name-like) to preserve that information for later pipeline steps.

Avoid going through an attribute assignment for an intermediate node result since it may not be the end result.

Closes https://github.com/cython/cython/issues/4268

Closes https://github.com/cython/cython/issues/4268

To be used in https://github.com/cython/cython/pull/4241

See https://github.com/cython/cython/pull/3626

Closes https://github.com/cython/cython/issues/4172

Allowing these gives people the false impression that they do something meaningful.

Closes https://github.com/cython/cython/issues/887
Closes https://github.com/cython/cython/issues/3959

People often have unrealistic expectations about what this typing helps with, so I've added a small paragraph to try to make it clearer.

The directives are called `optimize` rather than `optimization`.

1) Entry utility_code wasn't being imported unless the entry was visibly used. The utility code is needed for the definition even if unused. I think this is what utility_code_definition is for, but I'm not completely sure. I also had to add some calls to ensure it's used.

2) CppOptionalTempCoercions weren't being moved correctly. Tested by printing from the destructor.

3) cdef-class attributes weren't being created or destroyed correctly. Tested by printing from the destructor