LambdaFormEditor should use a transform lookup key that is not a SoftReference

LambdaFormEditor caches the LambdaForm result of some transforms using a Transform, which is both used as a lookup key and to hold a soft reference to the cached LambdaForm.

The lookup key is always discarded (a new one wrapping the LambdaForm to be cached will be allocated on a miss). This means we are queuing up a SoftReference. A non-SoftReference lookup key would also be thinner, causing less allocation pressure.