Closes #15162: the meaning of "online" has shifted a bit in recent years, use "interactive" instead.

only once (float obviously doesn't set any context at all).

plain tuples to immutable iterable objects with named attributes
(structseq objects).

measure.

that was last accessed. In the pi benchmark (64-bit platform, prec=9),
_decimal is now only 1.5x slower than float.