Performance could be better

[kevinburke]

Probably we can be smarter with allocations and buffer growing etc. to improve performance.

$ make bench
go list ./... | grep -v vendor | xargs go test -benchtime=2s -bench=. -run='^$' 2>&1 | /Users/kevin/bin/benchstat /dev/stdin
name         time/op
Copy-4          131µs ± 0%
Writer-4        152µs ± 0%
WriterBig-4     149µs ± 0%

name         speed
Copy-4       62.4MB/s ± 0%
Writer-4     54.0MB/s ± 0%
WriterBig-4  54.8MB/s ± 0%

name         alloc/op
Copy-4         47.7kB ± 0%
Writer-4       47.2kB ± 0%
WriterBig-4    14.4kB ± 0%

name         allocs/op
Copy-4            991 ± 0%
Writer-4          985 ± 0%
WriterBig-4       984 ± 0%