32bit integer overflow in stdlib_io_npy_save.fypp and load.fypp

[degawa]

When compiling stdlib using NAG Fortran compiler 7.0 for Windows on Windows 10, it terminates compiling with the error "Integer (KIND=3) overflow" (KIND=3 corresponds to int32 in NAG Fortran).
Errors are detected in to_bytes_i4 in stdlib_io_npy_save.fypp and in get_descriptor in stdlib_io_npy_load.fypp .

In the procedures, the integer literal 2**32 is used in expressions as below:

stdlib/src/stdlib_io_npy_save.fypp

Lines 63 to 66 in 8eaa778

	str = achar(mod(val, 2**8)) // &
	& achar(mod(val, 2**16) / 2**8) // &
	& achar(mod(val, 2**32) / 2**16) // &
	& achar(val / 2**32)

stdlib/src/stdlib_io_npy_load.fypp

Lines 135 to 143 in 8eaa778

	if (major > 1) then
	header_len = ichar(buf(1)) &
	& + ichar(buf(2)) * 2**8 &
	& + ichar(buf(3)) * 2**16 &
	& + ichar(buf(4)) * 2**32
	else
	header_len = ichar(buf(1)) &
	& + ichar(buf(2)) * 2**8
	end if

I guess that 2**24 is probably correct instead of 2**32.

I have understood that to_bytes_i4 converts an integer to a byte string by converting a 32 bit decimal integer to a base-256 number and then to four ASCII characters.
In this case, the power of base number (=256) at each position are (256**0, 256**1, 256**2, 256**3), corresponding to (2**0, 2**8, 2**16, 2**24).

This consideration does not cover negative integers, but if my understanding is correct, the concerning expressions can be modified as follows:

        str = achar(mod(val, 256**1)) // &
            & achar(mod(val, 256**2) / 256**1) // &
            & achar(mod(val, 256**3) / 256**2) // &
            & achar(val / 256**3)

and

        if (major > 1) then
            header_len = ichar(buf(1)) &
                &      + ichar(buf(2)) * 256**1 &
                &      + ichar(buf(3)) * 256**2 &
                &      + ichar(buf(4)) * 256**3
        else
            header_len = ichar(buf(1))  &
                &      + ichar(buf(2)) * 256**1
        end if

If using 2**32 is a trick to handle negative integers, I need another solution to compile stdlib using NAG Fortran compiler.

[awvwgk]

I think the proposed patch looks fine. Feel free to open a pull request and we can include it.