Add intern.Table.Query

[mcy]

Suppose we have m map[intern.ID]T and s string. We want to query if this string is in the map. We could write m[table.Intern(s)], but if s is not already interned, this is quite wasteful! Interning a never-before-seen string costs three map hits, plus the hit to m. However, if s is not interned, it cannot possibly be in m, so we can cut the cost of every lookup to one hit in Query and one in m[id].

In other words, this lets us write this function:

func get(tab intern.Table, m map[intern.ID]T, s string) (T, bool) {
  id, ok := tab.Query(s)
  if !ok { return T{}, false }
  
  v, ok := m[id]
  return v, ok
}

char6-inlined IDs are treated as always being interned, which means that for very small IDs, we only have to hit one map: the lookup in m[id].