opening brace should be on new line after broken impl line

[tspiteri]

Rustfmt currently produces this:

impl ImplementationXxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    for TypeXxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx {
    fn foo() {}
}

I think it looks better, and more consistent with other code, like this:

impl ImplementationXxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
    for TypeXxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
{
    fn foo() {}
}