equiv_make: Add -make_assert option

This adds a -make_assert flag to equiv_make. When used, the pass generates
$eqx and $assert cells to encode equivalence instead of $equiv.

tests/various/equiv_make_make_assert.ys

@@ -0,0 +1,32 @@
+read_verilog <<EOT
+module gold(
+	input wire [7:0] a,
+	input wire [7:0] b,
+	output wire [7:0] c
+);
+
+wire [7:0] b_neg;
+assign b_neg = -b;
+assign c = a + b_neg;
+endmodule
+
+module gate(
+	input wire [7:0] a,
+	input wire [7:0] b,
+	output wire [7:0] c
+);
+
+wire [7:0] b_neg;
+assign b_neg = ~b + 1;
+assign c = a + b_neg;
+endmodule
+
+EOT
+
+equiv_make -make_assert gold gate miter
+
+select -assert-count 0 t:$equiv
+select -assert-count 2 t:$assert
+
+prep -top miter
+sat -prove-asserts -verify