Writing support

[jkrumbiegel]

This PR continues the work in #46

It allows writing Tables.jl tables into the supported file formats. Metadata support is currently restricted to passing a file label only. Long strings over the threshold of 2045 bytes are currently not read back in correctly and I can't test if the proprietary software packages targeted would read these. I was also surprised that missing strings are read back in as "" but that might be intended. Only Stata files support the file types Int8, Int16, Int32, Float32, all others seem to convert these to Float64.

@vjd @MichaelHatherly @andreasnoack

[davidanthoff]

The idea of this package here is that it is a low-level wrapper around the C library that itself doesn't take any dependency on either Tables.jl or TableTraits.jl or anything like that. The idea is also that this package here typically won't be used by end-users directly, in the realm of Queryverse the end-user package really is https://github.com/queryverse/StatFiles.jl, and that is the package that for example brings the integration with TableTraits.jl along.

So ideally this package here would continue to not take a dependency on either Tables nor TableTraits, but instead just expose relatively low-level functions to write files and stay a package with as few dependencies as possible. And we can then add user-facing APIs to either StatFiles.jl, or any other package if someone wants to provide a more Tables.jl centric experience, and then those user-facing packages can share the implementation in this package here.

[jkrumbiegel]

I'm closing this PR for the time being, as our dependency ReadStatTables.jl has moved to directly using readstat_jll, therefore it doesn't make sense from our end anymore to add this code to ReadStat.jl.