Write a C package in Go

```go
package C

import "github.com/goplus/lib/c"

func Xadd(a, b c.Int) c.Int {
    return a + b
}

func Double(x float64) float64 {
    return 2 * x
}
```

is equivalent to

```c
int add(int a, int b) {
    return a + b;
}

double Double(double x) {
    return 2 * x;
}
```
