Auto merge of #139 - ollie27:bigint_hash, r=cuviper
Auto merge of #139 - ollie27:bigint_hash, r=cuviper

bigint: simplify Hash

There cannot be any leading zeros in a BigUint so just derive Hash which will just hash the Vec directly.
Add Hash to Sign so we can derive it for BigInt as well.

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