Refactor iter adapters with less macros

[camsteffen]

Just some code cleanup. Introduced a util and_then_or_clear for each of chain, flatten and fuse iter adapter impls. This reduces code nicely for flatten, but admittedly the other modules are more of a lateral move replacing macros with a function. But I think consistency across the modules and avoiding macros when possible is good.

[rust-highfive]

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[rust-highfive]

r? @joshtriplett

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[joshtriplett]

Looks like a win to me.

@bors r+

[bors]

📌 Commit 6587dda has been approved by joshtriplett

[scottmcm]

@bors try @rust-timer queue

(Just to check that the closures don't make things worse, and that we don't need to do something like #62429)

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

🙅 Please do not try after a pull request has been r+ed. If you need to try, unapprove (r-) it first.