fold_first iterator consumer

[conradludgate]

how many iterator adapters/consumers are we going to add...

rust-lang/libs-team#157

This simple extension on fold is very similar to reduce, but on numerous occasions I have wanted to reduce but into a different type. This means I manually re-implement reduce with fold every time I need that.

This implements a fold_first (please bikeshed) which provides an init method. reduce is re-implemented as fold_first(identity, f)

Similarly, I've implemented the try/rfold variants too

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

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[Noratrieb]

You need an ACP first.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.debug-assertions := True
configure: rust.overflow-checks := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
Attempting with retry: make prepare
---
   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0405]: cannot find trait `Residual` in this scope
   --> library/core/src/iter/traits/double_ended.rs:356:22
    |
356 |         R::Residual: Residual<Option<R::Output>>,
    |
help: consider importing this trait
    |
1   | use crate::ops::Residual;
1   | use crate::ops::Residual;
    |

error[E0412]: cannot find type `ChangeOutputType` in this scope
    |
    |
350 |     ) -> ChangeOutputType<R, Option<R::Output>>
    |
help: consider importing this type alias
    |
    |
1   | use crate::ops::ChangeOutputType;

error[E0433]: failed to resolve: use of undeclared type `FromResidual`
   --> library/core/src/iter/traits/double_ended.rs:364:38
    |
    |
364 |             ControlFlow::Break(r) => FromResidual::from_residual(r),
    |                                      ^^^^^^^^^^^^ use of undeclared type `FromResidual`
help: consider importing this trait
    |
1   | use crate::ops::FromResidual;
    |

[bors]

☔ The latest upstream changes (presumably #108421) made this pull request unmergeable. Please resolve the merge conflicts.