Progress towards faster `sortperm` for Strings

[nalimilan]

@ChrisRackauckas

You suggested that DataFrames.jl or other data readers could use interned strings so that way the performance is specialized to both domains. I think that’s a really great idea and would be a great use of Julia’s type system since most/all algorithms should work no matter what string type is internally used, and that context switch is a great way to get performance. I would be interested in seeing that investigated in more depth.

We kind of do this already since CSV.jl creates CategoricalArray vectors for columns with a small proportion of unique values. I think it makes sense to use CategoricalArray/PooledArray to represent values with lots of duplicates, and arrays of strings for “real” strings which are almost all different from one another.