Make macro-flags work outside of dataframesmeta macros

[pdeffebach]

I complain about R's nonstandard evaluation a lot because it's hard to inspect objects outside of their scope.

But I'm doing the same thing in DataFramesMeta with @passmissing, @byrow, and soon @astable. I should write @passmissing, @byrow, etc. so that they produce either a pair or a collection of pairs that someone can inspect.

This is motivated by discussions on discourse and this issue in DataFrames.jl.

cc @nathanrboyer