Inconsistent formatting of function signature based on where it appears

[topecongiro]

Function signatures are formatted in a different way, based on where it appears. For example,

fn foo(
    a: SomeLongComplexType,
    b: SomeOtherLongComplexType,
) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>> {
}

type MyFn = fn(a: SomeLongComplexType, b: SomeOtherLongComplexType)
    -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>>;

The latter case should be formatted like the former.

cc rust-lang/style-team#134