Avoid forced copy in step_setnames() and rename_with()

[tinygreen]

step_setnames() and rename_with() will now not force a copy() if lazy_dt() was called with immutable = FALSE. In line with how mutate() and relocate() already behave.

Added test for rename() with immutable = FALSE. Updated expected result of pivot_wider() test, as the call to step_setnames() triggered an unnecessary copy() there too, which is now also avoided.

Fixes #485