Implement structs as a value wrapped around a container type

[diamondburned]

Go structs generated should be defined as such:

type Record struct {
	*record
}

type record struct {
	native *C.CType
}

Then, the finalizer can be set like so:

v := &record{native: c}
runtime.SetFinalizer(v, (*record).free)

return &Record{
	record: v,
}

This way, the caller could safely copy Record without worrying about losing
the finalizer. It will also allow the generated Go code to express the same
level of pointers, though this might be trickier than it seems.

The generated record should have a Copy() method explicitly for duplicating
the underlying C value, though, and the method should reset the finalizer, but
some record types already have a _copy() method, so we don't actually need
that.