Draft: trial better ode wrapper

[oscardssmith]

This is a prototype of a replacement for FunctionWrappersWrappers specialization of ODEs. It works by not specializing the function, and wrapping the p and t fields to prevent them from needing to be boxed. This still has some overhead, and doesn't work for derivatives with respect to t or p, but it will work with arbitrary chunksizes for the jacobians.

julia> prob = ODEProblem{true}(lorenz!, [1.0; 0.0; 0.0], (0,100.0)) # FunctionWrapperWrapper
julia> @btime  solve(prob, Tsit5(), save_everystep=false);
  221.112 μs (74 allocations: 3.98 KiB)
julia> prob2 = ODEProblem(lorenz!, [1.0; 0.0; 0.0], (0,100.0)) # new wrapper
julia> @btime  solve(prob2, Tsit5(), save_everystep=false);
  254.728 μs (61 allocations: 3.48 KiB)