multiplication algorithms are not well tested

[make-github-pseudonymous-again]

Should add random 27 x 27 bits operands on byte arrays tests for mul and karatusba. Could extend to other operations like add or div.

the goal is to have the correct answer on 53 bits using byte arrays (UInt8Array).

[make-github-pseudonymous-again]

migrated to arithmetic-operations-for/naturals-big-endian#29