Hi!

I found src/eval.c (let) has redundant conditions, that compares
the length of the list with the current index and also checks if
the current list is cons.

I remove latter check, and it compiled fine and passed all the
tests I added.

However, in such cases, Lisper prefers to compare that the
current list is cons, rather than comparing indices.  Taking a
`cdr` and checking that it is cons is a Lisp idiom in situations
where `mapc` and `dolist` are not available.

The concern is it may be faster to check the index than CONSP.
This would be fast enough because CONSP in C is a macro, which is
eventually converted to a bitwise operation.  The code is
considered to be easier to read and less prone to bugs than the
current code that includes variable reuse and reassignment.

Regards,
Naoya.