Fix cast precedence and associativity based on updated F# spec operator precedence table

```
The F# 2.0 spec was recently updated and has changed the entry for :> and :?> 
from being the same as :? to having lower precedence and right associativity.
```

Original issue reported on code.google.com by `stephen....@gmail.com` on 13 May 2012 at 11:15
