Review usages of Integer#size in the gem

From @XrXr in https://bugs.ruby-lang.org/issues/20614#note-8, we should review usages of Integer#size as it is not equal to pointer size on Windows.

Line 10 in cadc8de

FIXNUM_BIT_SIZE = (0.size * 8) - 2

Lines 7 to 8 in cadc8de

    
           MIN_VALUE = -(2**(0.size * 8 - 2)) 
        
           MAX_VALUE = (2**(0.size * 8 - 2) - 1)

Line 32 in cadc8de

if 0.size == 4

	MIN_VALUE = -(2*(0.size 8 - 2))
	MAX_VALUE = (2*(0.size 8 - 2) - 1)