Fix closures

[devmotion]

In line with SciML/DelayDiffEq.jl#130, we should also fix all closures in DiffEqBase (and possibly other JuliaDiffEq packages) that access closed over variables since apparently it's behaviour is undefined. This issue appears, e.g., in the constructor of DiffEqFunctions (https://github.com/JuliaDiffEq/DiffEqBase.jl/blob/master/src/diffeqfunction.jl#L269 and similar lines) and in the root finding of callbacks (lines such as https://github.com/JuliaDiffEq/DiffEqBase.jl/blob/master/src/callbacks.jl#L417-L420).

[ChrisRackauckas-Claude]

Triage: DiffEqBase now uses FastClosures.@closure throughout, and the specific files/code referenced in this issue have been extensively reworked. Recommend closing.