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

tests/test_validation_errors/fixedpoint_enum.rdl

@@ -0,0 +1,15 @@
+addrmap top {
+    reg {
+        enum test_enum {
+            zero = 2'b00;
+            one = 2'b01;
+            two = 2'b10;
+            three = 2'b11;
+        };
+        field {
+            sw = rw; hw = r;
+            fracwidth = 0;
+            encode = test_enum;
+        } fixedpoint_enum[2] = 0;
+    } r1;
+};