A new approach of building strings

Instead of maintaining a buffer, this thing minimizes the amount of allocations by calling the user-provided function two times: the first time it sums the lengths of the strings returned using a callback by the function, the second time it pushes said strings into a String that was extended using the lengths sum.

An example

assert_eq!(
    build(|mut collector| {
        collector.collect(&"a");
        collector.collect(&"bcd");
        collector.collect(&"ef");
    }),
    "abcdef"
);