printer: use tabwriter to align instructions (#8)

README.md

@@ -65,10 +65,10 @@ After running `go generate` the [`add.s`](examples/add/add.s) file will contain
 
 // func Add(x uint64, y uint64) uint64
 TEXT ·Add(SB), NOSPLIT, $0-24
-	MOVQ	x(FP), AX
-	MOVQ	y+8(FP), CX
-	ADDQ	AX, CX
-	MOVQ	CX, ret+16(FP)
+	MOVQ x(FP), AX
+	MOVQ y+8(FP), CX
+	ADDQ AX, CX
+	MOVQ CX, ret+16(FP)
 	RET
 ```
 
@@ -137,18 +137,20 @@ The result from this code generator is:
 
 // func Sum(xs []uint64) uint64
 TEXT ·Sum(SB), NOSPLIT, $0-32
-	MOVQ	xs_base(FP), AX
-	MOVQ	xs_len+8(FP), CX
-	XORQ	DX, DX
+	MOVQ xs_base(FP), AX
+	MOVQ xs_len+8(FP), CX
+	XORQ DX, DX
+
 loop:
-	CMPQ	CX, $0x00
-	JE	done
-	ADDQ	(AX), DX
-	ADDQ	$0x08, AX
-	DECQ	CX
-	JMP	loop
+	CMPQ CX, $0x00
+	JE   done
+	ADDQ (AX), DX
+	ADDQ $0x08, AX
+	DECQ CX
+	JMP  loop
+
 done:
-	MOVQ	DX, ret+24(FP)
+	MOVQ DX, ret+24(FP)
 	RET
 ```