This fixes some theoretical bugs whereby we were using hashCode to try
to get unique keys for objects, but really we wanted object identity.
It also lays the groundwork for a new GlobalKey concept.
I tried to keep the impact on the code minimal, which is why the "Key"
constructor is actually a factory that returns a StringKey. The code
has this class hierarchy:
```
KeyBase
|
Key--------------+---------------+
| | |
StringKey ObjectKey UniqueKey
```
...where the constructors are Key and Key.stringify (StringKey),
Key.fromObjectIdentity (ObjectKey), and Key.unique (UniqueKey).
We could instead of factory methods use regular constructors with the
following hierarchy:
```
KeyBase
|
LocalKey---------+---------------+
| | |
Key ObjectIdentityKey UniqueKey
```
...with constructors Key, Key.stringify, ObjectIdentityKey, and
UniqueKey, but I felt that that was maybe a more confusing hierarchy.
I don't have a strong opinion on this.
This change makes it easier to defined only the width or the height of an image
and let the other value be filled in from the image's intrinsic aspect ratio.
Fixes#175
