`undef` and `todo`

[eelstork]

For top down design undef() should be useful but, although how it is designed makes sense in theory in practice I do not find a use.

• A default of failing vs dithering between values would make more sense.
• An option to assign a fixed value would be good.

The dithering model may go away. Until then provide options as above.

todo proposed as a variant. undef is designed to break production builds (no change). todo would issue a warning instead.

[eelstork]

There is not enough incentive to preserve the Undef decorator; will only keep newer APIs as above.

[eelstork]

Document new undef API before merging.