Allow buffer reuse in ReadHeader/WriteHeader variants.

[iamtakingiteasy]

Currently each ReadHeader/WriteHeader call incurs allocation penalty. This MR allows user to provide own buffer to those calls by adding ReadHeaderBuffer/WriteHeaderBuffer variants.

Benchmarks:

ReadHeader

goos: linux
goarch: amd64
pkg: github.com/gobwas/ws
cpu: AMD Ryzen 7 PRO 8840U w/ Radeon 780M Graphics  
BenchmarkReadHeader
BenchmarkReadHeader/no-mask#0
BenchmarkReadHeader/no-mask#0-16         	54530907	        21.27 ns/op	      16 B/op	       1 allocs/op
BenchmarkReadHeader/reused-buffer-no-mask#0
BenchmarkReadHeader/reused-buffer-no-mask#0-16         	152444149	         7.811 ns/op	       0 B/op	       0 allocs/op
BenchmarkReadHeader/mask#1
BenchmarkReadHeader/mask#1-16                          	46440010	        23.27 ns/op	      16 B/op	       1 allocs/op
BenchmarkReadHeader/reused-buffer-mask#1
BenchmarkReadHeader/reused-buffer-mask#1-16            	74732881	        14.33 ns/op	       0 B/op	       0 allocs/op
BenchmarkReadHeader/mask-u16#2
BenchmarkReadHeader/mask-u16#2-16                      	46687669	        23.77 ns/op	      16 B/op	       1 allocs/op
BenchmarkReadHeader/reused-buffer-mask-u16#2
BenchmarkReadHeader/reused-buffer-mask-u16#2-16        	72172458	        15.00 ns/op	       0 B/op	       0 allocs/op
BenchmarkReadHeader/mask-u64#3
BenchmarkReadHeader/mask-u64#3-16                      	42360039	        24.57 ns/op	      16 B/op	       1 allocs/op
BenchmarkReadHeader/reused-buffer-mask-u64#3
BenchmarkReadHeader/reused-buffer-mask-u64#3-16        	71174119	        15.51 ns/op	       0 B/op	       0 allocs/op
PASS

WriteHeader

goos: linux
goarch: amd64
pkg: github.com/gobwas/ws
cpu: AMD Ryzen 7 PRO 8840U w/ Radeon 780M Graphics  
BenchmarkWriteHeader
BenchmarkWriteHeader/no-mask
BenchmarkWriteHeader/no-mask-16         	80543956	        14.90 ns/op	      16 B/op	       1 allocs/op
BenchmarkWriteHeader/reused-buffer-no-mask
BenchmarkWriteHeader/reused-buffer-no-mask-16         	338765588	         3.562 ns/op	       0 B/op	       0 allocs/op
BenchmarkWriteHeader/mask
BenchmarkWriteHeader/mask-16                          	78015270	        15.51 ns/op	      16 B/op	       1 allocs/op
BenchmarkWriteHeader/reused-buffer-mask
BenchmarkWriteHeader/reused-buffer-mask-16            	294887298	         3.994 ns/op	       0 B/op	       0 allocs/op
BenchmarkWriteHeader/mask-u16
BenchmarkWriteHeader/mask-u16-16                      	70865414	        15.52 ns/op	      16 B/op	       1 allocs/op
BenchmarkWriteHeader/reused-buffer-mask-u16
BenchmarkWriteHeader/reused-buffer-mask-u16-16        	277524866	         4.278 ns/op	       0 B/op	       0 allocs/op
BenchmarkWriteHeader/mask-u64
BenchmarkWriteHeader/mask-u64-16                      	72674092	        15.71 ns/op	      16 B/op	       1 allocs/op
BenchmarkWriteHeader/reused-buffer-mask-u64
BenchmarkWriteHeader/reused-buffer-mask-u64-16        	274714274	         4.246 ns/op	       0 B/op	       0 allocs/op
PASS