Add safe translation for unions

[lucic71]

C/C++ unions are translated as a byte buffer + accessors that return a reinterpreted pointer into the byte buffer.

This C/C++ union:

union basic {
  int i;
  float f;
};

becomes:

pub struct basic {
    __bytes: Value<Box<[u8]>>,
}
impl basic {
    pub fn i(&self) -> Ptr<i32> {
        (self.__bytes.as_pointer() as Ptr<u8>).reinterpret_cast()
    }
    pub fn f(&self) -> Ptr<f32> {
        (self.__bytes.as_pointer() as Ptr<u8>).reinterpret_cast()
    }
}

Where __bytes is the byte buffer and i() and f() are the accessors that return reinterpreted views into the backing buffer.

Compared to Rust unsafe/C/C++ unions, this implementation is safe because it always serializes/deserializes the bytes into valid values or panics otherwise.

[nunoplopes]

I think Box::from([0u8; {}]) is more efficient

[lucic71]

They should be the same: https://godbolt.org/z/z59EYY6Wb. Historically Box::from([0u8; {}]) made a stack allocation for the array then copied the array on heap. But that seems to be solved so I will use your suggestion

[lucic71]

Done

[nunoplopes]

a lot of these formats are wasteful. It's better to do StrCat("impl Clone for", record_name)

[lucic71]

Done