switch to getcolumn and support tables in ND StructArray

[piever]

Now foreachfield relies on Tables.getcolumn.

Multidimensional StructArray are also treated as tables.

TODO

• Fully support Tables interface for LazyRows as well? (left for later PR)

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by 4.95%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   94.59%   89.63%   -4.96%     
==========================================
  Files          10       10              
  Lines         407      386      -21     
==========================================
- Hits          385      346      -39     
- Misses         22       40      +18

Impacted Files	Coverage Δ
src/StructArrays.jl	100% <ø> (ø)	⬆️
src/lazy.jl	96.42% <100%> (-3.58%)	⬇️
src/utils.jl	85.96% <100%> (-7.98%)	⬇️
src/tables.jl	75% <83.33%> (-9.62%)	⬇️
src/interface.jl	83.33% <0%> (-16.67%)	⬇️
src/collect.jl	95.77% <0%> (-4.23%)	⬇️
src/structarray.jl	85.56% <0%> (-6.44%)	⬇️
src/groupjoin.jl	86.66% <0%> (-4.25%)	⬇️
... and 1 more

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