Feature request: Allow `Symbol`s as `viewKey`s.

[dylanpyle]

Haven't dug into the code enough to know if there's a technical reason behind it, but it seems like extending the type of viewKey from string | undefined to at least string | symbol | undefined would be nice!

Use case here is that I have a well-known shared viewKey I'd like to use (i.e. not a resource ID of any kind) and I like Symbols over constant strings.

[dylanpyle]

Realizing one downside is that this won't easily be serializable to/from JSON, so maybe this is harder than just extending the type.

[scotttrinh]

The basic structure is already pretty hard to serialize, so, yeah, kinda tough. Using a string constant seems fine since it's rare you'll share that key outside of your internal application, but if we come up with a good reason to support it, I'm open to PRs!