Lazily init the cache for interface contracts.
Lazily init the cache for interface contracts.

This reduces the memory usage of very small classes.
It appears to have no real effect on large programs
such as DrRacket.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket