refactor: avoid capturing this in delegate using local self pointer

[gorsing]

To avoid implicit capturing of this in a delegate,
replaced direct field access with access via a local pointer self cast to Impl*.

Old similar and alternative #10816

This is a preventive fix for a potential Phobos error related to delegate lifetime,
as seen in: dlang/dmd#21514

No functional behavior change is intended.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10821"

[0xEAB]

Thanks for your contribution, @gorsing :)

Would you mind adding a unittest that showcases the advantage of this change? (Assuming that this is possible. If not, don’t bother.) Doing so could also potentially prevent accidental breakage caused by future changes.

[gorsing]

@0xEAB Changes aimed at eliminating leaks in closures
The behavior has not changed.
Unfortunately, I haven’t been able to come up with a visual test yet.
Perhaps dlang/dmd#21514 will be adopted in some form