`effect` and `observable` difference

[dy]

• effect is destination of connection graph, it is assigned as fx(callback, inputs). Other names - writable(?), destination(?), tap, sink.
• observable is source of connection graph, it is assigned to some data source. Other names - source, readable.

The point of this issue is stabilize naming. Observable seems to conflict with rxjs, observable proposal, npm observable, async iterables.

[dy]

Ok, pick effect and observable for now.