make 6g constants behave as ken proposes.  (i hope.)
various bug fixes and tests involving constants.

test/const1.go is the major new test case.

R=ken
OCL=26216
CL=26224
diff --git a/test/const.go b/test/const.go
index 85c0a91..8e587cf 100644
--- a/test/const.go
+++ b/test/const.go
@@ -14,6 +14,9 @@
 	c1 = chuge >> 100;
 	c3div2 = 3/2;
 	c1e3 = 1e3;
+
+	ctrue = true;
+	cfalse = !ctrue;
 )
 
 const (
@@ -111,4 +114,7 @@
 func main() {
 	ints();
 	floats();
+
+	assert(ctrue == true, "ctrue == true");
+	assert(cfalse == false, "cfalse == false");
 }