lift should support non-observables

[aplavin]

Currently, lift(f, x) only works if x is an observable, but errors (or does a completely different thing, because lift happens to be === map) for regular x values.

But it would be natural and convenient to make lift(f, x::Observable) = current behavior, and lift(f, x::Any) = f(x). This would enable preparing plotting arguments without manually handling observable and non-observable cases, like

# works no matter if x is a regular value or observable
y = lift(myfunc, x)
plot(y)