Add support for stdarg.h

[lucic71]

This PR adds support for va_list, va_start, va_arg, va_end, va_copy.

va_list is implemented as libcc2rs::VaList, a cursor over a slice containing VaArgs.

pub enum VaArg {
    Int(i32),
    UInt(u32),
    Long(i64),
    ULong(u64),
    Double(f64),
    RawPtr(*mut c_void),
    Ptr(AnyPtr),
}

pub struct VaList<'a> {
    args: &'a [VaArg],
    pos: usize,
}

impl<'a> VaList<'a> {
    pub fn arg<T: VaArgGet>(&mut self) -> T {
       ...
    }
}

Callers of variadic functions build an array of VaArgs, for example:

int sum_ints(int first, ...);
sum_ints(1, 2, 3, 4, 0)

becomes in unsafe

pub unsafe fn sum_ints_0(mut first: i32, args: &[VaArg]) -> i32
sum_ints_0(1, &[2.into(), 3.into(), 4.into(), 0.into()])

Then, a va_start call creates a new VaList:

va_list ap;
va_start(ap, n);

let mut ap: VaList = <VaList>::default();
ap = VaList::new(args);

va_end is a no-op, va_arg translates to VaList::arg, and va_copy translates to cloning the VaList.