Closure: Got rid of redundant error check.
Closure: Fix bool representation in error message.

All Lisp values except for boolean convert to string automatically.
Rather than inserting lisptostring everywhere there might be a boolean
maybe I should bite the bullet and represent #t and #f with objects
rather than true and false.
lambda with multi-expression body.

The Seasoned Schemer introduces lambda with a multi-expression body,
i.e. implicit "begin", on page 93.  (It introduces "let" with a
multi-expression body on page 78 and notes in a footnote on page 79 that
the same works with letrec and letcc.)