Make user defined object cache more reliable

[Takashi Kato]

  /* TODO: 
     The library we look up here might not have the class.
     ex) #<(sagittarus regex)> 
         (library (test) (export rx) (import (rnrs))
           (define rx #/this won't be valid/))
     The sample code will be valid for caching but reading will be invalid
     because we don't have any <pattern> class info anywhere. How should we
     treat this problem?
   */

This comment should be implemented since SgClass has now library field to get the actual binding.

[Takashi Kato]

Making user defined object cache more robust (Fixes #261)

→ <<cset 1c793033a017>>