Update bigdecimal files from 3.1

These will eventually be sourced from the gem.

ruby/bigdecimal#169

lib/ruby/stdlib/bigdecimal/ludcmp.rb

@@ -76,13 +76,13 @@ def lusolve(a,b,ps,zero=0.0)
       end
       x <<= b[ps[i]] - dot
     end
-    (n-1).downto(0) do |i2|
+    (n-1).downto(0) do |i|
       dot = zero
-      psin = ps[i2]*n
-      for j in (i2+1)...n do
+      psin = ps[i]*n
+      for j in (i+1)...n do
         dot = a[psin+j].mult(x[j],prec) + dot
       end
-      x[i2]  = (x[i2]-dot).div(a[psin+i2],prec)
+      x[i]  = (x[i]-dot).div(a[psin+i],prec)
     end
     x
   end

lib/ruby/stdlib/bigdecimal/math.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: false
 require 'bigdecimal'
-require 'bigdecimal/util'
 
 #
 #--

lib/ruby/stdlib/bigdecimal/util.rb

@@ -43,7 +43,7 @@ class Float < Numeric
   #
   # See also BigDecimal::new.
   #
-  def to_d(precision=Float::DIG+1)
+  def to_d(precision=0)
     BigDecimal(self, precision)
   end
 end