Rename the NaN and is_NaN methods to lowercase.

This is for consistency in naming conventions.

- ``std::num::Float::NaN()`` is changed to ``nan()``;
- ``std::num::Float.is_NaN()`` is changed to ``is_nan()``; and
- ``std::num::strconv::NumStrConv::NaN()`` is changed to ``nan()``.

Fixes #9319.