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_fixedpoint/regblock.rdl

@@ -0,0 +1,39 @@
+addrmap top {
+    default accesswidth = 64;
+    default regwidth = 64;
+    reg {
+        field {
+            sw = rw; hw = r;
+            intwidth = 8;
+            fracwidth = 8;
+        } f_Q8_8[16] = 0;
+        field {
+            sw = r; hw = w;
+            intwidth = 32;
+        } f_Q32_n12[20];
+        field {
+            sw = rw; hw = r;
+            fracwidth = 32;
+            is_signed;
+        } f_SQn8_32[24] = 0;
+        field {
+            sw = rw; hw = r;
+            fracwidth = 7;
+            is_signed;
+        } f_SQn6_7 = 0;
+    } r1 @ 0x0;
+
+    reg {
+        field {
+            sw = r; hw = w;
+            is_signed;
+        } f_signed[16];
+        field {
+            sw = rw; hw = r;
+            is_signed = false;
+        } f_unsigned[16] = 0;
+        field {
+            sw = r; hw = w;
+        } f_no_sign[16];
+    } r2 @ 0x8;
+};