parse-integer parses an integer in the specified radix
from the substring of string delimited by start and end.
parse-integer expects an optional sign (+ or -) followed by
a a non-empty sequence of digits to be interpreted in the specified radix.
Optional leading and trailing whitespace1 is ignored.
parse-integer does not recognize the syntactic radix-specifier
prefixes #O, #B, #X, and #nR,
nor does it recognize a trailing decimal point.
If junk-allowed is false, an error of type parse-error is
signaled if substring does not consist entirely of the representation of a
signed integer, possibly surrounded on either side by whitespace1
The first value returned is either
the integer that was parsed,
or else nil if no syntactically correct integer
was seen but junk-allowed was true.
The second value is either
the index into the string of the delimiter that terminated the parse,
or the upper bounding index of the substring if the parse terminated at
the end of the substring (as is always the case if junk-allowed