Add Tables interface to RowIterator

[tkf]

This PR turns RowIterator into a proper table. It came up in JuliaData/TypedTables.jl#57 to support semantically row-oriented append! with column-oriented memory access for optimization. The idea is to use append!(dest, Tables.row(src)) to declare that you are adding rows in src to dest even when iterate(src) is not defined for column-wise. Example usage is: append!(Table(a=[1], b=[2]), Tables.rows((a=[3], b=[4]))).

[codecov-io]

Codecov Report

Merging #126 into master will decrease coverage by 0.64%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
- Coverage   96.51%   95.87%   -0.65%     
==========================================
  Files           7        7              
  Lines         430      436       +6     
==========================================
+ Hits          415      418       +3     
- Misses         15       18       +3

Impacted Files	Coverage Δ
src/fallbacks.jl	95.04% <50%> (-2.85%)	⬇️

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