status: speculative, no RFC

Specific improvements being used in the shiny future section include:

Concise HRTB syntaxpermit T: Iterable<Item<'_> = &u32> or T::Item<'_>: Send instead of for<'a> T: Iterable<Item<'a> = &'a u32>
[HRTB implied bounds]The for<'a> syntax in HRTB means "any suitable 'a" and not "any 'a at all"
PoloniusPolonius-style borrow checking
Default outlives boundsAdd default bounds for where Self: 'a when appropriate rather than requiring users to write them automatically; add those same defaults to the impl