Which speeds it up at least twofold for a typical 32-bytes write (and that's
for a very naïve test that allocates new BufBinWriter on every iteration):
pkg: github.com/CityOfZion/neo-go/pkg/io
BenchmarkWriteBytes-8 10000000 124 ns/op
BenchmarkWriteBytesOld-8 5000000 251 ns/op
The logic here is that we'll have all binary encoding/decoding done via our io
package, which simplifies error handling. This functionality doesn't belong to
util, so it's moved.
This also expands BufBinWriter with Reset() method to fit the needs of core
package.
