avoid std::prelude imports #3029

Job	Run time
Cargo Package	18s
Format	14s
Cargo Deny	25s
Clippy	46s
MSRV	15s
Check for documentation errors (beta)	19s
Measure coverage	1m 19s
Validate external types appearing in public API	21s
Feature Powerset	16s
Check for documentation errors (stable)	17s
Check cross compilation targets	40s
Check semver compatibility	18s
Verify that no-std modes do not rely on libstd (--no-default-features --features alloc)	16s
Check for documentation errors (nightly)	16s
Build+test (--no-default-features --features alloc, ubuntu-latest, stable)	1m 3s
Build+test (--all-features, ubuntu-latest, stable)	1m 8s
Build+test (--no-default-features, ubuntu-latest, stable)	1m 7s
Verify that no-std modes do not rely on libstd (--no-default-features)	18s
Build+test (ubuntu-latest, stable)	1m 3s
Build+test (--features alloc, ubuntu-latest, stable)	1m 6s
	11m 45s

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