Initial intertask cancellation

Taskloops store tasks_done, tasks can be cancelled, and if a task named "killer" is in tasks_done then any other tasks are cancelled.

tests/intertask/killer.sby

@@ -0,0 +1,39 @@
+[tasks]
+killer
+aborted
+
+[options]
+mode bmc
+depth 100
+expect fail,cancelled
+
+[engines]
+killer: btor btormc
+aborted: smtbmc boolector
+
+[script]
+killer: read -define MAX=7
+aborted: read -define MAX=14
+read -formal demo.sv
+prep -top demo
+
+[file demo.sv]
+module demo (
+  input clk,
+  output reg [5:0] counter
+);
+  initial counter = 0;
+
+  always @(posedge clk) begin
+    if (counter == 15)
+      counter <= 0;
+    else
+      counter <= counter + 1;
+  end
+
+`ifdef FORMAL
+  always @(posedge clk) begin
+    assert (counter < `MAX);
+  end
+`endif
+endmodule