golang: Enforce maximum decoding depth

[2opremio]

See stellar/go#5057 for an example of the generated output

Depends on stellar/go-xdr#18

[tsachiherman]

Looks good. One thing I'd consider improving is the error objects -
Instead of dynamically creating the error object, i.e.

= errors.New(\"decoding #{name}: maximum decoding depth reached\")"

we could pre-created global error objects containing this object. then, we would return these objects. Creating an error of a specific struct would also allow us to further identify the error if that being triggered.
i.e.

type ErrorMaxDepthReached struct {
    objName string
}
func (o *ErrorMaxDepthReached) Error() string { ... }