Improve performance of FromObject and parseUUID

[sgmiller]

FromObject had a bug where it was attempting to preallocate a slice with
enough capacity to avoid growth, by calculating length^3. But ^ is XOR, so
it was allocation an empty slice (or at least much smaller than expected).

parseUUID is a bit of a odd function, parsing partial UUIDs as well as
correct ones. Simplified it to improve perf by 60%.

[banks]

Awesome. LGTM.

I checked the UUID index tests which do an OK job of testing different cases we expect to parse or not and this passes so seems like a solid improvement!

Is it worth adding a case to the existing test that exercises the invalid Hex path? It doesn't seem to have any input that would make it through hexToByte with an invalid input?

[Neal]

typo:

Suggested change

	// Convert to a []byte with a trailing nul byte
	// Convert to a []byte with a trailing null byte