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query: allow to always request the size alongside KeysOnly, allow impl to always return the size if no perf cost

Rational is that some datastore can easilty get the size of a value when
listing only keys, so we might as well have a way to pass that. One way this
could be useful is when implementing a cache for Has/GetSize.

Add clarifying comments on Query#String()

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>

Add a large test suite

Make it possible to print datastore queries.

doc: add a lead maintainer

feat: make not-found errors discoverable

Now we just need a common place to put:

func IsNotFound(e error) bool {
  for e != nil {
    if ne, ok := e.(interface { NotFound() bool }); ok {
      return ne.NotFound()
    }
    if ue, ok := e.(interface { Unwrap() error }); ok {
      e = ue.Unwrap()
    } else {
      return false
    }
  }
  return false
}

feat: make delete idempotent

fixes #104

Misc Typo Fixes

MapDatastore: obey KeysOnly

fix the keytransform datastore's query implementation

We use this datastore in tests all the time so it should match the behavior of
_actual_ datastores as much as possible.

The namespace Datastore shouldn't have to touch the mess with the query.

Note: this _also_ correctly applies ordering, filters, etc. to the transformed
keys (in both the namespaced and the key transform datastores).

Motivation: I want to be able to use the concrete type from a different package
but I can't.

This:

1. Exports the concrete datastore type and avoids returning private types from
   public functions.
2. Removes the mostly useless ktds interface. Unfortunately, it collides with
   the idiomatic name for the datastore itself.

This could break something referencing the `ktds.Datastore` type but nothing is
doing that.

sync: apply entire query while locked

filter: values are now always bytes