Currently, as referenced in SciML/DifferentialEquations.jl#971, the old
implementation of `handle_callbacks!` directly calls. `apply_callback!`
on `continuous_callbacks[idx]`, which is inherently type-unstable
because `apply_callback!` is specialized on the callback type.

This commit adds a generated function `apply_ith_callback!` which
generates type-stable code to do the same thing, where for each callback
tuple type, the generated function unrolls the tuple by checking the
callback index against static indicies. As a nice bonus, this generated
function seems to often be converted into a switch statement at the LLVM
level:
```
   switch i64 %4, label %L46 [
    i64 9, label %L3
    i64 8, label %L8
    i64 7, label %L13
    i64 6, label %L18
    i64 5, label %L23
    i64 4, label %L28
    i64 3, label %L33
    i64 2, label %L38
    i64 1, label %L43
  ]
```

For testing, I added an allocation test which sets up a simple ODE
problem, steps the integrator manually to before the first callback,
then manipulates integrator state past the first callback point. This
way, we can directly call `handle_callbacks!` and write a test on the
allocation count. I confirm that (at least testing against commit
SciML/DiffEqBase.jl@1799fc3, the current master branch tip in
DiffEqBase.jl), the new method does not allocate, whereas the old one
allocates. This may not be the case until a new release is cut of
DiffEqBase.jl, because the old version of
`find_first_continuous_callback` might allocate.