drop timeunit and timescale (closes #31)

test/basic/time.sv

@@ -0,0 +1,21 @@
+`timescale 1ns / 10ps
+timeunit 100ps;
+timeprecision 1ps;
+module top_1;
+    `timescale 1ns / 10ps
+    timeunit 1ps;
+    timeprecision 1ps;
+endmodule
+module top_2;
+    timeunit 100ps / 10fs;
+endmodule
+module top_3;
+    timeunit 10.0ps / 10fs;
+endmodule
+module top_4;
+    timeunit 100ps;
+    timeprecision 10fs;
+endmodule
+module top;
+    initial $display("Hello!");
+endmodule

test/basic/time.v

@@ -0,0 +1,3 @@
+module top;
+    initial $display("Hello!");
+endmodule