Use optimized observed interface

[YingboMa]

Currently, SciMLBase doesn't make use of the batched observed API at all, e.g. sol[[x, y]] should call observed([x, y], ...). This makes observed generation a lot slower than necessary. Also, when generated observed is present, it should always be safe to put actual states, parameters, and independent variables, so there's no need to split them.

Another optimization opportunity that SciMLBase misses is the constant use of the point-wise API even for arrays. Since observed is inherently type unstable, it's much more efficient to create a function barrier and call it like

obs = observed(syms)
obs.(us, ps, ts)

, and we should avoid calling it like

observed.((syms,), us, ps, ts)

[AayushSabharwal]

SII handles this now