fix signed/unsigned compilation issues in Vec

cc1plus: warnings being treated as errors
Vec.h: In function ‘int main(int, char**)’:
Vec.h:616: error: assuming signed overflow does not occur when assuming that (X + c) < X is always false
Vec.h:616: error: assuming signed overflow does not occur when assuming that (X + c) < X is always false

Alan says:

It is a result of optimization in the call sequence in test_append that calls str.append(value,len) which calls reserve(length() + count). length() is inlined to a reference to n so the argument is treated as "n+count". This yields, in the reserve method at the bad line

if (n+count <= n)

which leads to the warning/error you see. I don't see how to disable the error, though.