Better error message when no method missing can be found
Introduce ConstantTable

This is modelled after MethodTable and can be used to private constants
can be added for 1.9 mode. This only replaces the uses of the
LookupTable with the new MethodTable, it does not implement private
constants yet.
Implement public / private constants

This passes all the current specs found in Rubyspec regarding private
constants and when they should and should not be accessible.
Remove tags for passing private constant specs