refactor: save all reusable code generation logic

[korenmiklos]

Bitmask Building, dataframe copying, variable rewrites should all happen separately.

I propose a struct with

• df::Any
• local_copy::Symbol
• sdf::Symbol
• gdf::Symbol
• setup::Vector{Expr}
• teardown::Vector{Expr}

This could be created with one function call for all rewrite. Maybe with options, ifable, byable, etc.

Then we can go directly to the core logic of each rewrite.

quote $(Expr(:block, setup...)) Core logic here $(Expre(:block, teardown...)) end |> esc

[korenmiklos]

Done in #69