Add position to Enumerate

[x4exr]

In situations where the iterator progresses due to a function call, it may be tedious to determine the current position/count of the iterator.

fn do_iter(iter: &mut Enumerate<Chars<'a>>) {
    // ...
}

fn main() {
    do_iter(&mut i);

    // An iterator being used for parsing may want to compare the original position to the end position.
}

[rustbot]

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[rust-log-analyzer]

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#16 2.845 Building wheels for collected packages: reuse
#16 2.846   Building wheel for reuse (pyproject.toml): started
#16 3.093   Building wheel for reuse (pyproject.toml): finished with status 'done'
#16 3.094   Created wheel for reuse: filename=reuse-4.0.3-cp310-cp310-manylinux_2_35_x86_64.whl size=132715 sha256=dfa09868353292d98f811d3efdb0d54d07389e808efc71d68e3b93c514bf8bec
#16 3.095   Stored in directory: /tmp/pip-ephem-wheel-cache-e_kns945/wheels/3d/8d/0a/e0fc6aba4494b28a967ab5eaf951c121d9c677958714e34532
#16 3.097 Installing collected packages: boolean-py, binaryornot, tomlkit, reuse, python-debian, markupsafe, license-expression, jinja2, chardet, attrs
#16 3.492 Successfully installed attrs-23.2.0 binaryornot-0.4.4 boolean-py-4.0 chardet-5.2.0 jinja2-3.1.4 license-expression-30.3.0 markupsafe-2.1.5 python-debian-0.1.49 reuse-4.0.3 tomlkit-0.13.0
#16 3.493 WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv
#16 DONE 3.6s

[scottmcm]

If you want to add a new API to the standard library, please follow the instructions in https://std-dev-guide.rust-lang.org/development/feature-lifecycle.html#suitability-for-the-standard-library and start by making a case for the API in an ACP.

Then you can re-open this PR if the ACP is approved.