Refactor TableSelection

[juliohm]

This PR refactors the internal TableSelection type to avoid runtime checks in the wrong places. This type is intended for developers of TableTransforms.jl so the only check that should occur about the columns names is in the constructor. All further calls to Tables.getcolumn should assume the existence of columns for performance.

I also refactored the field type to a static tuple to improve the performance in downstream algorithm given the number of columns at compile time. It is also more consistent with other table types that return tuples in Tables.columnnames.

@eliascarv is there any particular reason we used Vector{Symbol} for the names or are we ok replacing it with tuple?

[eliascarv]

No particular reason. When the tests pass I will approve.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.30%. Comparing base (2ed7f99) to head (b1db9c0).

Files with missing lines	Patch %	Lines
src/tableselection.jl	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
+ Coverage   98.24%   98.30%   +0.06%     
==========================================
  Files          49       49              
  Lines        1424     1416       -8     
==========================================
- Hits         1399     1392       -7     
+ Misses         25       24       -1     

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[juliohm]

I will do an extra refactor to remove the oinds field. It is not needed either.