ICE on extremely deeply nested syntax

The following shell scripts all cause a stack overflow in the compiler:

``` sh
python -c 'print("("*100000 + ")"*100000)'| rustc /dev/fd/0
python -c 'print("{"*100000 + "}"*100000)'| rustc /dev/fd/0
python -c 'print("{("*100000 + "})"*100000)'| rustc /dev/fd/0
```

This should be fixed (perhaps by making the parser non-recursive, or by setting an explicit recursion limit).
