Improve padding performance

[kiyonlin]

Hey @muesli It's me again.

In this PR, I improve padding performance by reusing memory and decreasing memory allocation.

And I don't use Flush in String because we don't need cache to hold the result.

But yeah, MORE code again 😄

OLD:

BenchmarkPaddingString-8   	 3350841	       353 ns/op	     431 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3336720	       359 ns/op	     432 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3370746	       354 ns/op	     430 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3352352	       352 ns/op	     429 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3432604	       359 ns/op	     431 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3318981	       354 ns/op	     431 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3377637	       356 ns/op	     432 B/op	      12 allocs/op
BenchmarkPaddingString-8   	 3282550	       360 ns/op	     432 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3302713	       367 ns/op	     431 B/op	      11 allocs/op
BenchmarkPaddingString-8   	 3075331	       389 ns/op	     429 B/op	      11 allocs/op

NEW:

BenchmarkPaddingString-8   	 8087502	       147 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 8049085	       145 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 8362081	       143 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 7298126	       147 ns/op	       4 B/op	       1 allocs/op
BenchmarkPaddingString-8   	 7480929	       151 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 7328258	       146 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 7433443	       156 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 7692807	       155 ns/op	       4 B/op	       0 allocs/op
BenchmarkPaddingString-8   	 7617302	       162 ns/op	       4 B/op	       1 allocs/op
BenchmarkPaddingString-8   	 7181878	       162 ns/op	       4 B/op	       0 allocs/op

benchstat:

name             old time/op    new time/op    delta
PaddingString-8     357ns ± 3%     151ns ± 7%   -57.60%  (p=0.000 n=9+10)

name             old alloc/op   new alloc/op   delta
PaddingString-8      431B ± 0%        4B ± 0%   -99.07%  (p=0.000 n=10+10)

name             old allocs/op  new allocs/op  delta
PaddingString-8      11.0 ± 0%       0.0       -100.00%  (p=0.000 n=9+8)

[kiyonlin]

Gentle ping @muesli
When you have time, please review this PR and let me know your opinion 😄