Parsing "Microsoft encoding" is very lenient

I was reading through the code for Parse and I noticed that there is no validation on the first and last characters of the uuid-to-parse when the length of the input is 38 characters:

https://github.com/google/uuid/blob/16ca3eab7d2086fd5a82993a291cbf3b87fe38b7/uuid.go#L38-L61

I would expect there to be a check that `s[0] == '{' && s[37] == '}'` rather than simply ignoring those two characters. I am happy to send a PR if requested, it's a simple change. I am just verifying that this loose behavior is not actually desired.

(fwiw, it means that parsing something like `a01234567-abcd-cdef-abcd-012345678901a` would be parsed without an error, even though it has extra characters at the beginning and end. That's quite unexpected IMO.)