Add "vtbl" option for low-level interface vtable generation

[kennykerr]

The windows-bindgen crate's "sys" config option avoids generating bindings for COM interfaces since those necessarily require function bodies to implement virtual function calls, not directly supported by Rust. The "sys"-style bindings avoid generating function bodies to reduce build time.

However in some rare cases, it can be useful to have the ability to call COM interfaces without having to manually define them all from scratch or generating full-blown COM interface support. This update introduces the "vtbl" option as a middle ground that may be used along with the "sys" option and will instruct the windows-bindgen crate to generate the IID and vtable layouts as follows:

pub const IID_IStringable: GUID = GUID::from_u128(0x96369f54_8eb6_48f0_abce_c1b211e627c3);

#[repr(C)]
pub struct IStringable_Vtbl {
    pub base__: IInspectable_Vtbl,
    pub ToString: unsafe extern "system" fn(*mut ::core::ffi::c_void, *mut *mut ::core::ffi::c_void) -> HRESULT,
}

No functions are added, and you still have to go to the trouble of dereferncing a COM interface pointer, but at least the virtual function signatures are available for you to do so.

This is only available through the "vtbl" config option and does not affect the windows or windows-sys crates.