More allocator work #1505

Job	Run time
recursive	42s
nightly	1m 37s
clippy	48s
docs	30s
rustfmt	8s
no_default_features	11s
features (storage)	41s
features (descriptive)	16s
features (wire)	16s
features (json)	26s
features (value)	21s
features (serde)	21s
msrv	24s
test (--all-targets)	1m 49s
loom	2m 5s
test (--doc)	1m 35s
no_std (windows-latest, json)	45s
test (-p musli-core -p musli --doc --no-default-features)	51s
tests_clippy	1m 38s
no_std (windows-latest, serde)	49s
no_std (ubuntu-latest, json)	18s
no_std (ubuntu-latest, serde)	15s
miri	6m 48s
fuzz	5m 33s
	29m 7s

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