A lot of type instablities

[ChenNingCong]

I am not joking.
I recommend you to use my SimpleTypeChecker.jl to scan your files, which can capture typing error more easily.

1. Declare global variables in const.jl as const
white_keys, black_keys should be declared with const xxx = .... Otherwise they are inferred as Any because they can be assigned with arbitrary values. This impacts a lot of functions.
2. Reduce unnecessary use of .... Mostly of time, there is no necessary to convert an iterator to a Vector, because it supports iterator protocol.
3. findfirst at const.jl:335 return Union{Nothing, ...}. All find functions return Union..., it's a good habit to check nothing and prevent usage of a Union{Nothing}
4. In const.jl, const Fingering = SortedDict{Note, Finger} is an abstract type.
   Defined it as const Fingering = SortedDict{Note, Finger, Base.Order.ForwardOrdering}
5. In split.jl:77, split_length = ceil(notes_length / threads_num), ceil returns a Floating Number. You need to cast it to a Integer. Because it's reassigned with an Integer in the following code.

If you are unsure of what I have just said, please commit in this issue.