bigint: refactor BigInt ops
bigint: refactor BigInt ops

Try to let the underlying BigUint op implement val/ref forwarding as
much as possible, to reduce cloning.

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