typenamesym is called from three places:
typename, ngotype, and Type.Symbol.
Only in typename do we actually need a Node.
ngotype and Type.Symbol require only a Sym.
And writing the newly created Node to
Sym.Def is unsafe in a concurrent backend.
Rather than use a mutex protect to Sym.Def,
make typenamesym not touch Sym.Def.
The assignment to Sym.Def was serving a second purpose,
namely to prevent duplicate entries on signatlist.
Preserve that functionality by switching signatlist to a map.
This in turn requires that we sort signatlist
when exporting it, to preserve reproducibility.
We'd like to use Type.cmp for sorting,
but that causes infinite recursion at the moment;
see #19869.
For now, use Type.LongString as the sort key,
which is a complete description of the type.
Type.LongString is relatively expensive,
but we calculate it only once per type,
and signatlist is generally fairly small,
so the performance impact is minimal.