Templatize _d_interface_cast

[Albert24GG]

This PR introduces the templatized version of _d_interface_cast, making _d_cast the only hook used during lowering.

I have added a small walkaround in expressionsem.d to still get the cast lowered correctly when some implicit casts are being made. This may not be ideal, but I am open to suggestions.

I may also strip rt/cast_.d of unused functions, perhaps discarding the file completely.

[dlang-bot]

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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 + dmd#21473"

[teodutu]

@thewilsonator please take a look at this when you can

[thewilsonator]

make[1]: *** No rule to make target '/tmpfs/dautotest-work/repo/dlang.org/web/phobos-prerelease/rt_cast_.html', needed by 'doc'.  Stop.
rm import/core/sys/darwin/sys/attr.d

Not sure what you need to do to fix that.

[thewilsonator]

ah, you need to also remove https://github.com/dlang/dmd/blob/master/druntime/mak/DOCS#L545 and possibly also the corresponding entry inmak/COPY

[thewilsonator]

This otherwise is looking good