zero alloc: assume that works with inlining

[gretay-js]

This PR propagates [@zero_alloc assume] annotation from a function to its body, piggybacking on Debuginfo.t. The PR starts with a bit of refactoring to make it easier to add a field to Debuginfo.t. I'll be happy to separate the refactoring commit into another PR.

The propagation happens in middle_end during translation from Lambda and boils down to adding a new argument to Debuginfo.from_location and to the "environment" of translation.
This PR a draft because it looks too complicated. The change is separate for each middle end, and I am not sure if I have all the places updated correctly. Is there a better way? Would it be better to add another pass on Lambda dedicated to propagating this information?

Tested on small examples manually with all 3 middle ends. TODO: test on larger examples.

[gretay-js]

I'll split this up and rework the propagation, there are two approaches to try :

• propagate assume_zero_alloc when the function is inlined, instead of on the function body itself on the way out of lambda.
• propagate assume_zero_alloc at lambda, instead of doing it in the middle-end.. this means adding this field to Debuginfo.Scoped_location as well.