Support Lists of Variable-Length Structures.

It is currently possible to (somewhat hackily) support lists by modeling them as recursive structures, like so:

```
struct Repeated(remaining_size : UInt:32):
0 [+4] UInt length
# ... other fields ...
if remaining_size > length:
length [+remaining_size - length] Repeated next
```

However, there should be a nicer syntax for specifying that there is a list of `Repeated` elements, with a C++ API that supports iteration (but not random access).