More TODOs

[ErikQQY]

With #89 finally merged, let's push BoundaryValueDiffEq.jl to the next level!🎉🎉🎉🎉

Here are some issues we need to address in the near future:

• [x] The code style is a little weird, we need to use a better format style for the package

• [x] Currently, the solving speed is very slow and didn't use any special tricks for performance, we need to speed up the defect estimation process

• [x] The code is not optimized, we need to do a lot of refactors to make it match the DiffEq interface

• [ ] Add benchmarks for the existing BVP solvers