Add writing support

[TonyLianLong]

queryverse/StatFiles.jl#1

[davidanthoff]

Function probably has to look something like:

 function handle_!(data::Ptr, len::Int, ctx::Ptr)
          io = unsafe_pointer_to_objref(ctx)
          actual_data = unsafe_wrap(data, len)
          write(io, actual_data)
       end

[codecov-io]

Codecov Report

Merging #46 into master will decrease coverage by 20.03%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master      #46       +/-   ##
===========================================
- Coverage   78.63%   58.59%   -20.04%     
===========================================
  Files           2        2               
  Lines         117      157       +40     
===========================================
  Hits           92       92               
- Misses         25       65       +40

Impacted Files	Coverage Δ
src/ReadStat.jl	58.01% <0%> (-19.54%)	⬇️
src/C_interface.jl	61.53% <0%> (-22.68%)	⬇️

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[andreasnoack]

Would probably be best to add tests as well here. Is the PR otherwise ready?