Replace instance_state_parser with ResourceData interface

[jimmy-btn]

See func (d *ResourceData) State() *terraform.InstanceState { for a good example.

The core of this function is:

        rawMap := make(map[string]interface{})
	for k := range d.schema {
		source := getSourceSet
		if d.partial {
			source = getSourceState
			if _, ok := d.partialMap[k]; ok {
				source = getSourceSet
			}
		}

		raw := d.get([]string{k}, source)
		if raw.Exists && !raw.Computed {
			rawMap[k] = raw.Value
			if raw.ValueProcessed != nil {
				rawMap[k] = raw.ValueProcessed
			}
		}
	}

Within this, the call to d.get(...) which uses a combination of the Schema and InstanceState to build a nested and correctly typed representation of the resource.

Next steps are:

1. Work out how a ResourceData is constructed and initialized with the correct set of FieldWriters (MapFieldWriter, MergeFieldWriter etc)

2. Confirm that the representation that is returned works for lists and lists of nested resources

3. Write a class to convert from this representation to a Resource for printing