Replace num256 with bnum #2425

Job	Run time
Build without default features	1m 20s
Clippy (linter)	1m 13s
Test JNI(Android) bindings	10m 38s
Check intra-documentation links	2m 13s
Test Swift(iOS) bindings	3m 20s
Run JS tests and publish alpha versions	21m 45s
Code formatting (rustfmt)	19s
Tests on ubuntu-latest	5m 41s
Tests on windows-latest	15m 6s
Code coverage in tests	25m 21s
Tests on macOS-latest	5m 56s
	1h 32m 52s

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