Implemented binary marshalling/unmarshalling

[Andrew-Morozko]

I had a similar issue raised by #192

I want to use semver.Version in an encoding/gob serialized structure, so it needs to implement encoding.BinaryMarshaler and encoding.BinaryUnmarshaler.

I decided to use the fact that it's specifically a binary encoding and used varints from "encoding/binary". This makes binary marshaling 11.2x faster and unmarshaling 8.9x faster than text-based operations.

If you don't like low-level (although tested and straightforward) code you can implement Binary in terms of Text marshaling like so:

func (v Version) MarshalBinary() ([]byte, error) {
	return v.MarshalText()
}

func (v *Version) UnmarshalBinary(data []byte) error {
	return v.UnmarshalText(data)
}

[Andrew-Morozko]

Technically we can omit the last string length if we assume that the data slice is always correctly sized, but that seems to be a very niche optimization

[Andrew-Morozko]

Is the major-minor-patch-pre-metadata combo completely set in stone? If not, then perhaps we should throw in a "binary format version" number.

[Andrew-Morozko]

Or we can just copy the obvious 5-line long function from golang (if the license permits that, of course 😇)