feat: add optional `mimalloc` feature #22

Job	Run time
Build & Test (stable, ubuntu-latest)	53s
Build & Test (stable, windows-latest)	53s
Build Wasm	53s
Check fmt & build docs	37s
Minimal versions check (windows-latest)	39s
Build & Test (stable, macOS-latest)	56s
Minimal versions check (macOS-latest)	43s
Minimal versions check (ubuntu-latest)	40s
	6m 14s

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