initial pass improving decl parsing error messages

- all decl tokens are given an accurate starting position
- key grammar productions return token positions to facilitate the above
- helpers for standardized parse error generation
- replaced annoying pattern-matching type argument restrictions
- moving away from dumping raw decl tokens in error messages

test/error/enum_post_signed.sv

@@ -0,0 +1,6 @@
+// pattern: unexpected signed applied to enum
+module top;
+    enum {
+        A, B
+    } signed x;
+endmodule