Add rescue to support streamline error handling

This implements inline `rescue` like those in Ruby [1], but more powerful
in that users can specify the exception to be handled.

The `rescue` method will be very convenient in scripting.

[1] https://stackoverflow.com/questions/15396791/ruby-oneline-rescue

Author: liufengyun

tests/run/rescue.scala

@@ -0,0 +1,13 @@
+object lib {
+  inline def (op: => T) rescue[T] (fallback: => T) = try op catch { case _: Throwable => fallback }
+  inline def (op: => T) rescue[T, E <: Throwable] (fallback: E => T) = try op catch { case ex: E => fallback(ex) }
+}
+
+import lib._
+
+@main def Test = {
+  assert((9 / 1 rescue 1) == 9)
+  assert((9 / 0 rescue 1) == 1)
+  assert(((9 / 0 rescue { ex: NullPointerException => 5  }) rescue 10) == 10)
+  assert(((9 / 0 rescue { ex: ArithmeticException => 5  }) rescue 10) == 5)
+}