Source locations move off the attribute dict into a dedicated SrcRef on
AttrObject, interned in a per-design SrcPool that hash-conses sets of
location twines. Locations share their file prefix with each other through
the twine pool, and objects that fuse (opt_merge, share, ff/mem rewrites)
carry the union of their inputs' locations instead of a '|'-joined string.
write_rtlil emits an `srcs` section of location sets alongside `twines`;
read_rtlil resolves `\src "@N"` back to a handle and still accepts a plain
location string. -readable renders the locations as text.
Emil J. Tywoniak
ba74d22f6b