Initial implementation of elaboration system tasks

(IEEE1800-2017 section 20.11)
This PR allows us to use $info/$warning/$error/$fatal **at elaboration time** within a generate block.
This is very useful to stop a synthesis of a parametrized block when an
illegal combination of parameters is chosen.

tests/various/elab_sys_tasks.sv

@@ -0,0 +1,30 @@
+module test;
+localparam X=1;
+genvar i;
+generate
+if (X == 1)
+  $info("X is 1");
+if (X == 1)
+  $warning("X is 1");
+else
+  $error("X is not 1");
+case (X)
+  1: $info("X is 1 in a case statement");
+endcase
+//case (X-1)
+//  1: $warn("X is 2");
+//  default: $warn("X might be anything in a case statement");
+//endcase
+for (i = 0; i < 3; i = i + 1)
+begin
+  case(i)
+    0: $info;
+    1: $warning;
+    default: $info("default case statemnent");
+  endcase
+end
+
+$info("This is a standalone $info(). Next $info has no parameters");
+$info;
+endgenerate
+endmodule

tests/various/elab_sys_tasks.ys

@@ -0,0 +1 @@
+read_verilog -sv elab_sys_tasks.sv