Merge a lot of optimize from bytes.Buffer

[lrita]

Hi @syndtr ,

I merge a lot of optimize from bytes.Buffer, e.g.:

golang/go@5531040
golang/go@c08ac36
golang/go@1ba4556

name                       old time/op   new time/op   delta
WriteByte-4                 22.9µs ± 2%   14.8µs ± 1%  -35.54%  (p=0.008 n=5+5)
Alloc-4                     22.5µs ± 1%   18.8µs ± 0%  -16.23%  (p=0.004 n=5+6)
BufferNotEmptyWriteRead-4    276µs ± 2%    301µs ± 3%   +8.97%  (p=0.004 n=5+6)
BufferFullSmallReads-4      73.2µs ±24%   63.3µs ± 3%  -13.49%  (p=0.002 n=6+6)

name                       old speed     new speed     delta
WriteByte-4                179MB/s ± 2%  277MB/s ± 1%  +55.12%  (p=0.008 n=5+5)
Alloc-4                    182MB/s ± 1%  217MB/s ± 0%  +19.36%  (p=0.004 n=5+6)