maint: Add local function make_compound(conn, ...)

* dietncl/filter/prenorm.lua: New function make_compound(conn, ...);
Delete function capture_max_min; Fix comments.
* tests/test-filter-prenorm.lua: Likewise.
maint: Edit function make_condition_action_bijection(ncl)

* dietncl/filter/prenorm.lua: Likewise.