add FromBytes constructor

[loderunner]

FromBytes is a convenience constructor. It relies on UnmarshalBinary to create a new UUID. This makes creating a new UUID from a slice a bytes clearer in the client code without having to create a temporary variable, and more consistent with creating a slice by parsing a string.

Example:

s := "f47ac10b-58cc-0372-8567-0e02b2c3d479"
suuid, err := uuid.Parse(s)
b := []byte{
	0x7d, 0x44, 0x48, 0x40,
	0x9d, 0xc0,
	0x11, 0xd1,
	0xb2, 0x45,
	0x5f, 0xfd, 0xce, 0x74, 0xfa, 0xd2,
}
buuid, err := uuid.FromBytes(b)

---

I realize FromBytes is a bit of a name collision with ParseBytes. I am open to better suggestions for naming, of course. In fact, when discovering the library, I originally expected ParseBytes to perform this copy.

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[loderunner]

I signed it!

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[pborman]

Thanks!