Add a new Zap field constructor that encodes an array of zero or more
Zap-marshalable objects into a Zap array.

Usage:

    // Given,
    //
    // func (*User) MarshalLogObject(zapcore.ObjectEncoder) error

    zap.Objects("foo", []*User{u1, u2, u3})

Before this, users would have to resort to constructing a `[]*User`
wrapper, and use Zap's `Array` constructor:

    // Before:

    type users []*User

    func (uu users) MarshalLogArray(enc zapcore.ArrayEncoder) error a
        for _, u := range uu {
            if err := enc.AppendObject(u); err != nil {
                return err
            }
        }
        return nil
    }

    zap.Array("foo", users{u1, u2, u3})

Credit to ztstewart for the suggestion.

Refs GO-618

Reviewed-At: D7294727
Reviewed-By: ztstewart

In the previous change, we added a Objects field constructor to support
logging arrays of any object supported by zap.Object.

    func (*User) MarshalLogObject(...) error { ... }
    var users []*User = ...
    zap.Object("users", users)

That solution is incomplete because it does not cover the case when,
instead of a `[]*User`, we have a `[]User` despite the method being on
`*User`.

This change adds a similar ObjectValues field constructor that handles the
case where we have a `[]T` but the MarshalLogObject is on `*T`.

This works by declaring a new type constraint objectMarshalerPtr.
`objectMarshalerPtr[T]` is true for a type that is a pointer to `T`,
and implements zapcore.ObjectMarshaler.

Note that we still do not cover the case where the method is on the value
receiver (`func (User) MarshalLogObject(..) error`) and we have a
`[]*User`. We can add that when it becomes necessary.

Reviewed-At: D7295739
Reviewed-By: ztstewart

Minor optimization to avoid copying the struct to a variable.

Refs GO-618

Reviewed-At: D7297933
Reviewed-By: abhinav

staticcheck is expected to have support for generics-based code in a few
weeks. Disable it until then.