adds code generation for field comparisons

Adds new code generation functions to `pkg/generate/code` that output Go
code that compares fields of various types and calls `delta.Add()` if a
difference is detected.

For fields with underlying scalar types, the generated code looks like
this:

```go
if *a.ko.Spec.Name != *b.ko.Spec.Name) {
        delta.Add("Spec.Name", a.ko.Spec.Name, b.ko.Spec.Name)
}
```

For non-scalar types, generated code recurses over the fields and/or
calls specialized comparison functions in the runtime library's
`pkg/compare` module. Here is an example of a complex underlying field
type and what the generated code might look like:

```go
if ackcompare.HasNilDifference(a.ko.Spec.CreateBucketConfiguration, b.ko.Spec.CreateBucketConfiguration) {
        delta.Add("Spec.CreateBucketConfiguration", a.ko.Spec.CreateBucketConfiguration, b.ko.Spec.CreateBucketConfiguration)
} else {
        if ackcompare.HasNilDifference(a.ko.Spec.CreateBucketConfiguration.LocationConstraint, b.ko.Spec.CreateBucketConfiguration.LocationConstraint) {
                delta.Add("Spec.CreateBucketConfiguration.LocationConstraint", a.ko.Spec.CreateBucketConfiguration.LocationConstraint, b.ko.Spec.CreateBucketConfiguration.LocationConstraint)
        } else {
                if *a.ko.Spec.CreateBucketConfiguration.LocationConstraint != *b.ko.Spec.CreateBucketConfiguration.LocationConstraint {
                        delta.Add("Spec.CreateBucketConfiguration.LocationConstraint", a.ko.Spec.CreateBucketConfiguration.LocationConstraint, b.ko.Spec.CreateBucketConfiguration.LocationConstraint)
                }
        }
}
```

Author: jaypipes