Check for zero denominator when parsing Ratio
Check for zero denominator when parsing Ratio

Closes #122

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