Expand FlattenCompat folds

[cuviper]

The former chain+chain+fold implementation looked nice from a
functional-programming perspective, but it introduced unnecessary layers
of abstraction on every flat_map/flatten fold. It's straightforward
to just fold each part in turn, and this makes it look like a simplified
version of the existing try_fold implementation.

For the iter::bench_flat_map* benchmarks, I get a large improvement in
bench_flat_map_chain_sum, from 1,598,473 ns/iter to 499,889 ns/iter,
and the rest are unchanged.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

@bors try @rust-timer queue

r=me if perf looks reasonable (improved or unchanged)

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit fc150d1 with merge 031d8ee87cc71178b63641c7fce0155a5da0aacb...

[bors]

☀️ Try build successful - checks-actions
Build commit: 031d8ee87cc71178b63641c7fce0155a5da0aacb (031d8ee87cc71178b63641c7fce0155a5da0aacb)

[rust-timer]

Queued 031d8ee87cc71178b63641c7fce0155a5da0aacb with parent 83b30a6, future comparison URL.

[rust-timer]

Finished benchmarking try commit (031d8ee87cc71178b63641c7fce0155a5da0aacb): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[cuviper]

r=me if perf looks reasonable (improved or unchanged)

I think it does look like this went in the right direction, so...

@bors r=Mark-Simulacrum

[bors]

📌 Commit fc150d1 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit fc150d1 with merge 63bacf1...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 63bacf1 to master...