vecfold: Fold vectors of Results into Result of vector

This package provide a generic way to "unwrap" a Vec<Result<T,E>> into Result<Vec<T>, E>. Here is an example:

let valid: Vec<Result<u32, _>> = "1,2,3".split(",").map(|x| x.parse::<u32>()).collect();
let invalid: Vec<Result<u32, _>> = "1,2,a".split(",").map(|x| x.parse::<u32>()).collect();

// happy path, no errors, just the values
assert_eq!(vec![&1, &2, &3], valid.foldr().unwrap());

// sad path returns the error
assert!(invalid.foldr().is_err());

// happy path, no errors, return empty error vector
assert_eq!((vec![&1, &2, &3], vec![]), valid.foldr_bisect());

// sad path, populate error vector
let (ok, _) = invalid.foldr_bisect();
assert_eq!(vec![&1, &2], ok);