Cheapen index creation, speed up symbol column lookup
Cheapen index creation, speed up symbol column lookup

If columns groups come back, at least restrict storage to Int (!Real)

tshort/DataFrames.jl,emilbayes/DataFrames.jl,tkelman/DataFrames.jl,JuliaPackageMirrors/DataFrames.jl,pwoolcoc/DataFrames.jl,niloynibhochaudhury/DataFrames.jl,armgong/DataFrames.jl,scls19fr/DataFrames.jl,campanja-forks/DataFrames.jl,zhmz90/DataFrames.jl,JuliaStats/DataFrames.jl,ccphillippi/DataFrames.jl,anriseth/DataFrames.jl