codecgen: remove notfastpath from runtime tags for codecgen

Adding notfastpath to the runtime tags will cause the fastpath
methods to not be found. codecgen expects them to be available.

Fix by removing notfastpath from the tags.

Fixes #111

codec/codecgen/gen.go

@@ -220,7 +220,7 @@ func Generate(outfile, buildTag, codecPkgPath string, uid int64, useUnsafe bool,
 	os.Remove(outfile)
 
 	// execute go run frun
-	cmd := exec.Command("go", "run", "-tags=notfastpath "+goRunTag, frunMainName) //, frunPkg.Name())
+	cmd := exec.Command("go", "run", "-tags="+goRunTag, frunMainName) //, frunPkg.Name())
 	var buf bytes.Buffer
 	cmd.Stdout = &buf
 	cmd.Stderr = &buf