bigint: add tests for From implementations
bigint: add tests for From implementations

rust-num/num-integer,DonSheddow/num,jbcrail/num,koverstreet/num,anchovieshat/num,rust-num/num-traits,rust-lang/num,rust-num/num-bigint,rust-num/num-traits,rust-num/num-integer,rust-lang/num,rust-num/num,vks/num,rust-num/num-bigint,jbcrail/num,DonSheddow/num,anchovieshat/num,vks/num,koverstreet/num,rust-num/num