DictoVec: derive from AbstractVector

[alyst]

Sometimes R data.frame are constructed such that columns have element names.
RData.jl would then convert these columns into DictoVec object.
However, when converting R data.frame into DataFrames.jl DataFrame object, all column containers are required to support AbstractVector interface.
This PR fixes the failure of DataFrame construction by making DictoVec inherited from AbstractVector and adding necessary methods that are used during DataFrame construction.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (159f19e) 86.64% compared to head (8b00563) 86.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   86.64%   86.70%   +0.05%     
==========================================
  Files          14       14              
  Lines         689      692       +3     
==========================================
+ Hits          597      600       +3     
  Misses         92       92              

Files Changed	Coverage Δ
src/DictoVec.jl	96.25% <100.00%> (+0.04%)	⬆️

... and 2 files with indirect coverage changes

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