Number of bits in two's complement integer representation
For non-negative integers, (integer-length i) is the minimum
number of bits needed to represent the integer. Any integer can be
represented as a signed two's complement field with a minimum of
(+ (integer-length i) 1) bits.
The guard for integer-length requires its argument to be an
integer. Integer-length is defined in Common Lisp. See any Common Lisp
documentation for more information.