Allow ViewEnvironment (esp ViewRegistry) to support merging values

[rjrjr]

In kotlin, it's surprising that that envA + envB stomps A's ViewRegistry entirely. It's not hard to work around, but it's tedious.

We should think about providing a general mechanism to allow entries to merge, and make ViewRegistry use it.

[rjrjr]

Addressed in kotlin by square/workflow-kotlin#737