Add GCHeapMallocator

[pbackus]

In preparation for the removal of GCAllocator.deallocate and GCAllocator.reallocate, make that functionality available via a new allocator with a different name.

Users of GCAllocator that wish to continue using manual @system memory management instead of automatic @safe memory management can switch to GCHeapMallocator.

For the rationale behind these changes, see PR #8554 and issue 23318.

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CC @CyberShadow @schveiguy

@atilaneves this adds a new symbol

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	23318	enhancement	GCAllocator should not implement deallocate

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#8555"

[pbackus]

core.exception.AssertError@std/logger/core.d(1905): foo

Looks like a flaky test. Force pushing to retry...

[burner]

https://github.com/dlang/phobos/pull/8557

[pbackus]

Rebased to pick up #8557

[schveiguy]

Can this be an alias/wrapper underneath? Hard to review this when most likely it's simply a copy-paste of existing code.

[pbackus]

@schveiguy Made it a wrapper.

[atilaneves]

Please see my comments on the other PR.