add a round_ties_even for Ratio<T>

[m4rch3n1ng]

i am working on a rust scheme interpreter, and the scheme (round x) procedure is specified to "round[...] to even when x is halfway between two integers", and is supposed to work on all number types available in scheme, which includes rationals.

since that interpreter uses this library, so i figured i'd try to get my implementation into upstream.

the actual implementation i did seems "too easy", i just cmp it with a Ratio::new(one, two) and then do the same with Greater and Less like the round fn does it. if it is Equal i first round towards zero (via trunc, like in the Less path) see if it is_even, and then round away from zero (by adding/subtracting 1, like in the Greater path) if not.