Remove SpecialFunctions dep (or replace with Bessels)

[t-bltg]

I can't find why the SpecialFunctions dependency was introduced in #84.

SpecialFunctions is know to trigger lots of dependencies (see the discussions in JuliaMath/SpecialFunctions.jl#409).

If bessel functions are required in this package, could you please consider switching to the recent, pure julia https://github.com/JuliaMath/Bessels.jl ?

The @lazy approach in #182 is interesting, but:

1. it can not be dropped in as usual since SpecialFunctions types are used in the module itself;
2. it does not remove SpecialFunctions from the list of dependencies.

Cc @juliohm for JuliaPlots/UnicodePlots.jl#291.

[cormullion]

I think the Colors/ColorVectorSpace packages are short of maintainers at present. @kimikage has moved on, it seems, and @timholy is doubtless overworked; recruiting some more color specialists is needed!