Similar to `Transformed._build`, it now correctly returns the output of its `subcon` build.

Before this commit using `Restreamed` would fail when accessing the context as in the
following example:

```python
RestreamedPassthrough = lambda subcon: Restreamed(subcon, lambda b: b, 1, lambda b: b, 1, lambda n: n)

x = Struct(a=RestreamedPassthrough(Const(1, Byte)), b=Rebuild(Byte, this.a))
x.build({})
```

In the `Rebuild` context `a` was not populated and was `None`. With this fix it is correctly
populated with the built context of the sub-construct.