Delete a needless symbol.
Delete a duplicate exporting of atom.
Redefine nth, nthcdr. Now they can take abstract-lists. Be careful, the order of arguments are opposite to CL's.
Rename hash-table functions.

* hash-table-key-exists-p => hash-exists-p
* hash-table-keys => hash-keys
* hash-table-values => hash-values
(fixes #25) Add let1.