i64 is missing from types available in `defineStruct`.

[Tezza48]

I have been playing around with this library but have hit a blocker: i64 isn't a valid field type when using defineStruct.

For example, I would require a definition for tagMSG from "user32.dll" in "winuser.h" as follows:

const MSG = defineStruct([
    ["hwnd", "pointer"],
    ["message", "u32"],
    ["wParam", "u64"],
    ["lParam", "i64"],
    ["time", "u64"],
])

This is not possible due to i64 being missing.
I can probably work around this but it ought to be added.