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/stub.go

@@ -0,0 +1,7 @@
+// Code generated by command: go run asm.go -out cast.s -stubs stub.go. DO NOT EDIT.
+
+package cast
+
+// Split returns the low 64, 32, 16 and 8 bits of x.
+// Tests the As() methods of virtual general-purpose registers.
+func Split(x uint64) (q uint64, l uint32, w uint16, b uint8)