Cols to dollar

[pdeffebach]

With this PR, we no longer have @transform df :y = f(cols("x")), but rather @transform df :y = f($"x").

[pdeffebach]

Okay tests pass, so this is ready for a review.

There are two problems that I am fine resolving later, after this is merged.

1. For the purposes of tests, we still need cols occasionally, particularly in the function_compilation tests. Also, the @linq tests seem to error with $ inside a @testset. Overall, this is strong evidence we need to keep cols around permanently. If users want to use DataFramesMeta.jl macros inside other macros, cols is a great way to get around the mine-field that is interpolation with $ and it's many meanings.
2. Broadcasting. In general, you can write

x = rand(10); 
@. x - $mean(x)

But you can't do the same in DataFramesMeta.jl. @. x - ^($)mean(:x) does not even work.

This is not a big deal, I guess. You can't have $ work for every instance. But it's a bit of a bummer.

We should still merge, though.

[bkamins]

Do you know if the same issues with $ are in DataFrameMacros.jl?

[pdeffebach]

The same issues exist with DataFrameMacros.jl. I've spoken to @jkrumbiegel and he's fine with not working with every use-case of $ out there. We should probably accept that, too, given how many different meanings it has throughout the ecosystem.

It's probably something we can iterate on at a later date, possibly through nesting $$.

[nalimilan]

Looks good, just a minor comment. The issues you mentioned are annoying, it would be nice to find a way to escape/protect $ from other macros so that we don't have to keep cols forever.

[pdeffebach]

Yes. I don't have any ideas on how to do that at the moment, but I will ask people who are more experienced in metaprogramming. Maybe John Myles White can return to his roots and help DataFramesMeta.jl again.

[nalimilan]

Maybe John Myles White can return to his roots and help DataFramesMeta.jl again.

Actually John hasn't contributed to this package at all. It was created by Tom Short.

[pdeffebach]

Ah sorry! I got my packages confused.

I'm going to merge! This will create merge conflicts I will have to sort out in #267