Bump num-bigint from 0.4.4 to 0.4.6 #198

Job	Run time
Check (nightly)	14s
prek	22s
Check (stable)	12s
Check (1.63.0)	1m 42s
Test Suite (--features=serialize)	19s
Rustfmt	8s
Test Suite (--all-features)	21s
Build documentation	20s
Check semver compatibility	20s
Clippy	15s
Test Suite (--no-default-features)	14s
Validate external types appearing in public API	20s
Test Suite (--features=integers)	15s
	5m 2s

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