variable substitution and solved states not applied to symbolic callback expressions

[wsphillips]

We get orphaned symbolics in the function bodies of generated callback functions because symbolic callbacks don't apply substitutions and solved states generated during structural_simplify.

# nonsense example but demos the problem
using ModelingToolkit, OrdinaryDiffEq

@variables t x(t) y(t)
@parameters τ
D = Differential(t)
@named fol_separate = ODESystem([ y  ~ 1 - x, D(x) ~ y/τ]; discrete_events = [(y == 4.0) => [x ~ x + 10]])
fol_simplified = structural_simplify(fol_separate)
ModelingToolkit.get_discrete_events(fol_simplified) # still has y(t) state
prob = ODEProblem(fol_simplified, [x => 0.0], (0.0,10.0), [τ => 3.0]) # missing preface + substitution
sol = solve(prob, Tsit5()) # error

[wsphillips]

I realized this is a duplicate of #1396 and #1646 so I'm closing this one