cdb make exists

(make-make-exists cdbm key)

Returns true if a key exists in the constant database being created.

Example:

> (define cdbm (make-cdb "test.cdb"))
> (cdb-make-add cdbm "A" 1)
> (cdb-make-exists cdbm "A")
#t
> (cdb-make-exists cdbm "B")
#f
> (cdb-make-finish cdbm)
> (define cdb (init-cdb "test.cdb"))
> (cdb-find cdb "A")
1
> (cdb-find cdb "B")
#f
> (cdb-finish cdb)