Simplify macro rewriters and avoid some prospective bugs

[thautwarm]

You may want to check this clean implementation of @select:
https://github.com/thautwarm/Query.jl/blob/34f6dbaf5aba156fafa9d60335126a7795f6b676/src/table_query_macros.jl#L49

macro select(args...)
    foldl(args, init=NamedTuple()) do prev, arg
        @match arg begin
            :(everywhere()) => :_

            ::Int && if arg > 0 end =>
                :( merge($prev, QueryOperators.NamedTupleUtilities.select(_, Val(keys(_)[$arg]))) )

            ::Int && if arg < 0 end =>
                let sel = ifelse(prev == NamedTuple(), :_, prev)
                    :( QueryOperators.NamedTupleUtilities.remove($sel, Val(keys($sel)[-$arg])) )
                end
            ::QuoteNode =>
                :( merge($prev, QueryOperators.NamedTupleUtilities.select(_, Val($(arg)))) )
     ...
end

Also, there're some prospective bugs that've been avoided, e.g., quoted " in fieldname of dataframes.

df |> @select(startwith("\"a\""))

[thautwarm]

I have no idea about the compatibility issues when Julia version is lower than v1.0 :(

[davidanthoff]

I have no idea about the compatibility issues when Julia version is lower than v1.0 :(

I'll look through the rest of the PR when I have bit more time, but just a quick response on this: I think we could probably drop julia 0.7 support at this point. I think we should keep 1.0 support, given that that is the long term supported version of julia.

[thautwarm]

Perfect, I'd like to continue refactoring those macro rewriters.

[codecov-io]

Codecov Report

Merging #238 into master will increase coverage by 0.73%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   90.02%   90.75%   +0.73%     
==========================================
  Files           5        5              
  Lines         441      411      -30     
==========================================
- Hits          397      373      -24     
+ Misses         44       38       -6

Impacted Files	Coverage Δ
src/Query.jl	78.57% <ø> (ø)	⬆️
src/table_query_macros.jl	100% <100%> (+7.79%)	⬆️

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[thautwarm]

There seems to be some strange issues in Julia v1.0. I cannot solve it on my own so I went to community for help. https://discourse.julialang.org/t/strange-bug-segmentation-fault-11-solely-existed-in-julia-v1-0/21392
Also, I've already made total simplification for table_query_macros.jl, becoming capable of raising an error when users' input is not in expected case, which I think might be what you expect but don't have time to deal with.