Implement allocateZeroed for building-block allocators

[n8sh]

Followup to #6411

Some allocators can allocate zeroed memory more efficiently than allocate + memset.
[...]
This PR adds non-public allocateZeroed methods to Mallocator and MmapAllocator and GCAllocator. Various derived allocators could also implement such methods when their parent allocators support them, but that work is left to a future PR

[dlang-bot]

Thanks for your pull request, @n8sh!

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Testing this PR locally

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[wilzbach]

I like this and we should really start to expose allocateZeroed publicly. Maybe do so in a separate new PR?

[wilzbach]

There's a bit of duplication with allocate as essentially only the called subfunction differs.
Maybe generate the string for both and mix it in?