Simplify parameter binding for BSTR

[kennykerr]

BSTR is relatively unusual as an input parameter since it is generally unnecessary, but it happens and can make certain APIs harder to use. This update just treats BSTR as any other generic borrowed type. Practically that means that you can pass None instead of having to create an empty BSTR for optional parameters. For example, connecting to a WMI server is a lot simpler:

let server =
    locator.ConnectServer(&BSTR::from("root\\cimv2"), None, None, None, 0, None, None)?;