FPlot axis labels for `@p`

[jariji]

This has nice axis labels

@p begin
    StructArray(x=[1,2,3], y=[10,20,30])
    FPlot(__, (@o _.x), (@o _.y))
    axplot(scatter)(__)
end

But this doesn't.

@p begin
    StructArray(x=[1,2,3], y=[10,20,30])
    FPlot(__, _.x, _.y)
    axplot(scatter)(__)
end

It's not so bad and I'm sure there's a reason, but it would be a little nice if it worked.

[aplavin]

@p is a very simple syntax transform, one of the things it does it turning _.x into a -> a.x. Naturally, after this transformation one cannot extract nice labels anymore...

I've been thinking recently about having a @po macro that does the same as @p but turns _.x into @o _.x instead of a -> a.x. It's useful in several places, eg FPlot and SQLCollections.
It's very tempting to make @o the default for @p, but "optics" aren't a 100% replacement for "anonymous functions": basically, 1 in @o _.x + 1 is a runtime value, while 1 in a -> a.x + 1 is a compile-time constant. This may affect constprop.