Simplify `assume`/`observe` design

[ParadaCarleton]

Creating this issue after @yebai asked me to -- it looks to me like assume and observe could be simplified a bit.

Current thoughts:

1. We should clearly document the differences between observe, tilde_observe, tilde_observe!!, etc.
2. If it's possible, we could try and combine some.
3. Make the behavior of observe, assume, etc. more similar, e.g. by having both return the same values.

[devmotion]

Probably @torfjelde knows best why the current design was chosen, what could be simplified, and which changes are problematic.