Add support for tuples and better round tripping of named tuples

[palday]

JuliaCall currently commits some type piracy to provide support for tuples:

function sexp(x ::Tuple)
    r = sexp(Array{Any}([x...]))
    setattrib!(r, :class, sexp("JuliaTuple"))
    r
end

This PR upstreams a more efficient (and memory safe) approach that avoids the type piracy. The type piracy is particularly unfortunate because it breaks JellyMe4 when used from within JuliaCall. (JuliaCall works by starting Julia and running RCall from the Julia session.)

I also take advantage of the new tuple implementation to simplify the named tuple implementation a bit and potentially make it a bit more efficient.

Finally, I take advantage of R's class-via-attributes system to annotate the lists created from (named) tuples as coming from Julia, which makes it possible to default to constructing (named) tuples, i.e. the original types, when roundtripping them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (69c22dd) 75.23% compared to head (793e12f) 75.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   75.23%   75.76%   +0.53%     
==========================================
  Files          25       26       +1     
  Lines        1623     1642      +19     
==========================================
+ Hits         1221     1244      +23     
+ Misses        402      398       -4     

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[ararslan]

Makes sense AFAICT