In functions with hanging indent, put the opening paren on the previous line

E.g., 

```
fn foo <a, b, c> (
    a: A) {
}
// not
fn foo <a, b, c>
    (a: A) {
}
```

This is more apparent when both generics and arguments are on multiple lines. I'm not 100% sure which is better TBH, so we should probably keep an option for it. Interested on opinions on this too.
