maint: Add function make_compound_tree(ncl)

* dietncl/filter/prenorm.lua: New function turn_binary_unary(conn);
New function turn_unary_binary(conn, ncl, ...);
New function update_binds(attrR1, attrR2, conn, ncl, cond);
New function correct_isolation(conn, ncl).

* tests/test-filter-prenorm.lua: Likewise.
maint: Fix comments. Delete useless variables.

* dietncl/filter/prenorm.lua: Likewise.