Add signed/fixedpoint properties (#140)

* declared intwidth, fracwidth, and is_signed UDPs

* fix boolean type name in UDP definition

* generate hwif fields with fixedpoint indices

* make "counter" and "encode" properties mutualy exclusive with signed/fixedpoint

* add signed/unsigned to hwif

* improved fixedpoint error messages, added validation tests

* added fixedpoint tests

* fixedpoint/signed not allowed for signal components

* added signed/fixedpoint UDP docs

* handle single-bit fixedpoint numbers

* fix too many positional arguments lint

* changed spelling of fixedpoint to fixed-point

* use "logic" in place of "unsigned logic"

* split signed and fixedpoint docs, added examples

* allow enums with is_signed=false

* split signed and fixedpoint implementations

* assorted nits picked

* updated is_signed validation unit test

hdl-src/regblock_udps.rdl

@@ -36,3 +36,19 @@ property wr_swacc {
     component = field;
     type = boolean;
 };
+
+property is_signed {
+    type = boolean;
+    component = field;
+    default = true;
+};
+
+property intwidth {
+    type = longint unsigned;
+    component = field;
+};
+
+property fracwidth {
+    type = longint unsigned;
+    component = field;
+};