Problem:
When an evaluation {-item calls `nvim_eval_statusline()`, that nested
call may overwrite the same memory used for `stl_items`.
Solution:
Make `curitem` static and use it to compute an offset to avoid
overwriting `stl_items` in nested calls to `build_stl_str_hl()`. Move
miscellaneous statusline tests into `describe()` block.
87e806186c