datastore: Export query struct fields

[RPGillespie6]

Is your feature request related to a problem? Please describe.
I'm trying to use Redis to cache datastore entities and queries. The problem is that the Query struct does not export any of its fields (kind, ancestor, etc) so I have no way of generating a cache key from a provided query, other than using hacky reflection techniques or building a wrapper around it.

Describe the solution you'd like
Export fields that could be used to generate a redis key:

type Query struct {
	Kind       string
	Ancestor   *Key
	Filter     []filter
	Order      []order
	Projection []string

	distinct   bool
	distinctOn []string
	keysOnly   bool
	eventual   bool
	Limit      int32
	offset     int32
	start      []byte
	end        []byte

	namespace string

	trans *Transaction

	err error
}

Describe alternatives you've considered
Current best alternative is to build a wrapper around Query so a way to uniquely id query can be captured or to use hacky reflection (which I don't want to do since a later version of datastore may break it).

As an example:
Given a key datastore.Key I can generate a redis key with fmt.Sprintf("%s:%d", key.Kind, key.ID). With a query though... I have no way of uniquely fingerprinting it so it's hard to map a query to cached results of that query. If you don't want to expose those fields, maybe a Fingerprint() method would be fine as well.

i.e. if you had a q datastore.Query you could call q.Fingerprint() which returns a string that uniquely identifies it (could be as simple as kind:ancestor:filters:order:projection:limit, for example User:::Last Name::100)