Link code fragments to core/asm output

The Godbolt compiler explorer has this for asm. The idea is to let the playground UI indicate, in some fashion, which parts of the Core or assembly dump corresponds to which part of the source code. I'm not sure how exactly this should work for Core, but for assembly the relevant information is apparently present in info tables (suggestion due to Gergő Érdi).

This would be a sizeable change, adding significant amounts of code to both the worker and the web interface.

Review how the worker produces Core and assembly dumps and how the web interface displays them. Compare the Godbolt approach and investigate the assembly info tables mentioned in the issue; the work is done when the UI can indicate which dump fragments correspond to source code, with an agreed approach for Core.

Written by the indexing model from the issue text.