Fixes for the new step methods in the range.c & numeric.c

Hi,

an included patch for the step method in range.c is a simple
bug-fix - for example

---
class Class
  alias succ superclass
end

# currently throws an exception complaining that succ for nil
# is undefined
(Fixnum..Object).step(2) {|x| p x }
---

The patch for the step method in numeric.c adds a sequence 
end point adjustment strategy - #step_fuzzyness measures the
``fuzzyness'' of step-length additions around the end-point
and adds a safety margin (there exist probably much better 
step_fuzzyness measures) - in pure Ruby:  

---
class Float
    def test_step(b,s)
      n = ((b - self)/ s).round
      last = n*s + self
      
      fuzzyness = step_fuzzyness(n,s)
      if (s > 0 && last > b + fuzzyness) || \
         (s < 0 && last < b - fuzzyness)
         n-=1
         last = n*s + self
      end
      
      if (last - b).abs < fuzzyness
        last = b
      end
        
      (n-1).times {|i| yield i*s + self }
      yield last
	self
    end
    
    EPSILON = 2.0**(-43)
    def step_fuzzyness(n, s)
      tmp = ((n-1)..(n+1)).collect do |i| 
       ((i*s + self - ((i-1)*s + self)) - s).abs
      end
      tmp.push s.abs*EPSILON
      2*tmp.max
    end
end


# with this patch on my machine ...
M = 52343
T = 12418.979
K = 20300012
L = 15673

p b = (K/T)                  # => 1634.595887
p e = (-L*M + K)/T           # => -64423.31749
p s = -M/T                   # => -4.214758717
p err = M/T * (2.0**(-40))# => 3.677147842e-12

# with this patch on my machine

last = nil
b.step(e,s) {|last| }
p e - last       # => 0.0

b.step(e+err,s) {|last| }
p (e+err) - last # => -4.214758717

b.step(e-err,s) {|last| }
p (e-err) - last # => -7.275957614e-12


# a bigger error is necessary to demonstrate
# fuzzyness in pure Ruby 

p err = M/T * (2.0**(-38.00))

b.test_step(e,s) {|last| }
p e - last       # => 0.0

b.test_step(e+err,s) {|last| }
p (e+err) - last # => -4.214758717

b.test_step(e-err,s) {|last| }
p (e-err) - last # => -1.455191523e-11
---


/Christoph