[Truffle] Have a decent path for Time#to_f since that seems used much in practice.

* Time#-(Time) should be used instead.
* Or even better Process.clock_gettime(Process::CLOCK_MONOTONIC).

Author: eregon

truffle/src/main/ruby/core/rubinius/common/time.rb

@@ -173,7 +173,7 @@ def to_r
   end
 
   def to_f
-    to_r.to_f
+    seconds + nsec * 0.000000001 # Truffle: optimized
   end
 
   def +(other)