Intercept and replace symbols rather than function calls

[rdeits]

instead of turning x + y into _conditional(+, x, y), we could transform it into Var(x) + Var(y) and add appropriate overloads to the Var wrapper type. That would avoid the need to intercept and transform broadcasting operations, since we could just implement (<=)(x::Var, y::Var) = Conditional(<=, x, y) and get [x, x] .<= [y, y] for free.