Move `entropy` to StatsAPI.jl and import it from there?

[George Datseris]

Hi there, we are also defining and exporting entropy in ComplexityMeasures.jl in different context (by providing ways to discretize data into probability mass functions). Would be great if we can get the same entropy name. There is no conflict because we also define types for each discretization format.

Would you agree to put entropy into StatsAPI.jl and import it from there?

We would prefer to not add StatsBase.jl as a dependency as we do not use any of its functions (and furthermore, it depends on Missings.jl which we would like to avoid as the Julia language provides robust missing support already).