Use decimal_shift in log and exp calculation

Author: tompng

lib/bigdecimal.rb

@@ -174,7 +174,7 @@ def self.log(x, prec)
     if x > 10 || x < 0.1
       log10 = log(BigDecimal(10), prec)
       exponent = x.exponent
-      x = x * BigDecimal("1e#{-x.exponent}")
+      x = x._decimal_shift(-exponent)
       if x > 3
         x /= 10
         exponent += 1
@@ -240,7 +240,7 @@ def self.exp(x, prec)
     # exp(x * 10**cnt) = exp(x)**(10**cnt)
     cnt = x > 1 ? x.exponent : 0
     prec2 = prec + BigDecimal.double_fig + cnt
-    x *= BigDecimal("1e-#{cnt}")
+    x = x._decimal_shift(-cnt)
     xn = BigDecimal(1)
     y = BigDecimal(1)