Accommodate arith-overflow in core::num, std::num, coretest::num.

* `core::num`: adjust `UnsignedInt::is_power_of_two`,
   `UnsignedInt::next_power_of_two`, `Int::pow`.

   In particular for `Int::pow`: (1.) do not panic when `base`
   overflows if `acc` never observes the overflowed `base`, and (2.)
   if `acc` does observe the overflowed `base`, make sure we only
   panic if we would have otherwise (e.g. during a computation of
   `base * base`).

 * also in `core::num`: avoid underflow during computation of `uint::MAX`.

 * `std::num`: adjust tests `uint::test_uint_from_str_overflow`,
   `uint::test_uint_to_str_overflow`, `strconv`

 * `coretest::num`: adjust `test::test_int_from_str_overflow`.

src/libcoretest/num/mod.rs

@@ -92,7 +92,7 @@ mod test {
         assert_eq!("127".parse::<i8>().ok(), Some(i8_val));
         assert_eq!("128".parse::<i8>().ok(), None);
 
-        i8_val += 1 as i8;
+        i8_val = i8_val.wrapping_add(1);
         assert_eq!("-128".parse::<i8>().ok(), Some(i8_val));
         assert_eq!("-129".parse::<i8>().ok(), None);
 
@@ -100,7 +100,7 @@ mod test {
         assert_eq!("32767".parse::<i16>().ok(), Some(i16_val));
         assert_eq!("32768".parse::<i16>().ok(), None);
 
-        i16_val += 1 as i16;
+        i16_val = i16_val.wrapping_add(1);
         assert_eq!("-32768".parse::<i16>().ok(), Some(i16_val));
         assert_eq!("-32769".parse::<i16>().ok(), None);
 
@@ -108,7 +108,7 @@ mod test {
         assert_eq!("2147483647".parse::<i32>().ok(), Some(i32_val));
         assert_eq!("2147483648".parse::<i32>().ok(), None);
 
-        i32_val += 1 as i32;
+        i32_val = i32_val.wrapping_add(1);
         assert_eq!("-2147483648".parse::<i32>().ok(), Some(i32_val));
         assert_eq!("-2147483649".parse::<i32>().ok(), None);
 
@@ -116,7 +116,7 @@ mod test {
         assert_eq!("9223372036854775807".parse::<i64>().ok(), Some(i64_val));
         assert_eq!("9223372036854775808".parse::<i64>().ok(), None);
 
-        i64_val += 1 as i64;
+        i64_val = i64_val.wrapping_add(1);
         assert_eq!("-9223372036854775808".parse::<i64>().ok(), Some(i64_val));
         assert_eq!("-9223372036854775809".parse::<i64>().ok(), None);
     }