add C ffi for parsing XMB files

[ScanMountGoat]

The XMB data from BinRead can be mapped into types compatible with C. This can also be easily converted to classes in C#.

#[repr(C)]
struct Entry {
    name: CString,
    parent_index: i32,
    properties: FfiVec<Property>
}

#[repr(C)]
struct Property {
    name: CString,
    value: CString
}

#[repr(C)]
struct Xmb {
    entries: FfiVec<Entry>
}

#[repr(C)]
pub struct FfiVec<T: Sized> {
    ptr: *mut T,
    size: usize,
}