Prevent cbn from unfolding the 'heap_lang' term.

This MR adds Arguments heap_lang : simpl never. Whenever cbn was used before this addition, heap_lang got unfolded to a large term. This is especially insidious because the language term appears in the implicit argument of fupd, which means this unfolding is not visible by default.

In my project, the performance benefit of this change was quite substantial. I think other downstream projects that use cbn might also see some performance improvements.