Force observer and computed methods to be static

[theengineear]

This is related to #28. Also, it's potentially a nice way for us to remove a fork that may not be providing much benefit.

Benefits to making these static:

• no chance of naming collision for instance level properties
• one less fork in our code
• information can be used in static analysis since it doesn't depend on an instance

Downsides to making these static:

• a bit more restrictive
• observer callback may fell a bit less ergonomic since we'll change the call signature

For computed... There's not too much argument here, there should be no need to provide a target. Forcing these methods to be static will also help with potential, future memoization of computations.

For observer... We want to support access to the target still, so we would need to change the signature to be myCallback(target, value, oldValue)--or something like that.

[theengineear]

@klebba , we discussed this at one point as a "funny, terrible idea". I still think the pros outweigh the cons here, but we should have more discussion before implementing.