namespaces: make set-global/get-global foldable

namespaces: make set-global/get-global foldable
Store the globals hashtable as an array of boxes so that the key-to-reference mapping is constant. Use a singleton and an unfoldable "box-at" word so that get-global and set-global optimize to direct operations on the associated box when the variable name is a compile-time constant. Fixes #200.