Unify vm benchmark prefixes to vm_ (#3028)

The vm1_ prefix and vm2_ had had special meaning until
820ad9cb1d and
12068aa4e9. AFAIK there's no special
meaning in vm3_ prefix.

As they have confused people (like "In `benchmark` what is difference
between `vm1_`, `vm2_` and `vm3_`"), I'd like to remove the obsoleted
prefix as we obviated that two years ago.

benchmark/vm_backtrace.rb

@@ -0,0 +1,22 @@
+# get last backtrace
+
+begin
+  caller(0, 0)
+rescue ArgumentError
+  alias caller_orig caller
+  def caller lev, n
+    caller_orig(lev)[0..n]
+  end
+end
+
+def rec n
+  if n < 0
+    100_000.times{
+      caller(0, 1)
+    }
+  else
+    rec(n-1)
+  end
+end
+
+rec 50