Add support for mimalloc

[MarkusPettersson98]

This PR adds mi-malloc as an alternative memory allocator to libc. This is exposed via the new mimalloc feature flag in the gotatun-cli crate.

I've not played around too much with it yet, but adding the initial infrastructure for experimenting with different memory allocators proved to be rather straightforward.

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This change is 

[Serock3]

@Serock3 reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @hulthe)

[hulthe]

We should test whether this allocator also fragments before adding it imo. We wouldn't want to encourage something that doesn't perform well.

@hulthe reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MarkusPettersson98)