A tool to help concatenate, implemented with a consuming builder pattern.
```rust let welder = Welder::with_start(' ', "foo");
let welder = welder.push("bar"); let welder = welder.push("baz"); let welder = welder.push("boat");
let string: String = welder.weld();
assert_eq!("foo bar baz boat", &string); ```
```rust let welder = Welder::with_start(0, 12);
let vec: Vec<_> = welder.push(14) .push(16) .push(18) .weld();
asserteq!(&[12, 0, 14, 0, 16, 0, 18], vec.asslice()); ```