Use Tables.isrowtable (Tables v1.0 API)

[tkf]

This PR simplifies and improves Tables.jl integration by using Tables.isrowtable introduced by JuliaData/Tables.jl#134. This patch requires Tables v1.0 which is not released yet. So I'm posting it as a WIP PR for now.

This PR combined with JuliaData/TypedTables.jl#57 makes append!(::TypedTables.Table, ::StructArray) work column-by-column as an optimization.

[codecov-io]

Codecov Report

Merging #116 into master will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master    #116      +/-   ##
=========================================
- Coverage   95.62%   95.6%   -0.03%     
=========================================
  Files          10      10              
  Lines         389     387       -2     
=========================================
- Hits          372     370       -2     
  Misses         17      17

Impacted Files	Coverage Δ
src/tables.jl	100% <50%> (ø)	⬆️
src/interface.jl	100% <0%> (ø)	⬆️
src/utils.jl	98.21% <0%> (ø)	⬆️
src/groupjoin.jl	90.9% <0%> (ø)	⬆️
src/collect.jl	100% <0%> (ø)	⬆️
src/lazy.jl	100% <0%> (ø)	⬆️
src/structarray.jl	92% <0%> (ø)	⬆️
src/sort.jl	93.97% <0%> (ø)	⬆️

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