yul: include ingot id in function name

From codex in #1230:

> In multi-ingot builds, two functions with the same module path/name (e.g., both at the root) can still produce identical mangled names because function_qualifier only hashes module_path_segments and returns the literal "root" for empty paths. module_path_segments doesn’t include the ingot name, so the qualifier can’t distinguish between ingots; if the signatures also match, mangled_name will collide across ingots, breaking linking or causing the wrong function to be referenced. Consider folding the ingot identity into the qualifier/hash so cross-ingot duplicates remain unique.

Maybe we could remove some of the hash noise in our current naming scheme while we're at it. One possible strategy is to use `$` separators in function names, eg `ingot_name$$path$to$function_name$$generic_args`. (There may be cases where we need the hash, I haven't really thought this through)

Start by locating function_qualifier, module_path_segments, and mangled_name, then trace how they construct names in multi-ingot builds. Confirm the collision with identical functions across ingots; done means equivalent functions from different ingots always receive distinct mangled names and linking resolves the intended function.

Written by the indexing model from the issue text.