clang::Cursor::args should return an Option<Vec<Cursor>>

[fitzgen]

Right now it always returns a Vec<Cursor> but it makes FFI calls to clang_Cursor_getArgument which are only valid if the cursor's referent is a function or method declarations and calls:

http://clang.llvm.org/doxygen/group__CINDEX__TYPES.html#ga673c5529d33eedd0b78aca5ac6fc1d7c

The argument cursor can be determined for calls as well as for declarations of functions or methods. For other cursors and for invalid indices, an invalid cursor is returned.

We should check whether the cursor referent is a valid kind, and if it is not, we should return None. This will be one less foot gun.

I can mentor whoever would like to pick up this bug.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[Natim]

I will try to give it a try with @glasserc

[fitzgen]

Great!