reg: support for register casting

Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().

Closes #1

tests/cast/cast_test.go

@@ -0,0 +1,17 @@
+package cast
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out cast.s -stubs stub.go
+
+func TestSplit(t *testing.T) {
+	expect := func(x uint64) (uint64, uint32, uint16, uint8) {
+		return x, uint32(x), uint16(x), uint8(x)
+	}
+	if err := quick.CheckEqual(Split, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}