A common helper function is used for UUID.String(), UUID.URN(), and
UUID.MarshalJSON(). Any perforance hit in UUID.String() and
UUID.Marshal() appears to be negligable. The benefit to
UUID.MarshalJSON() is several hundred nanoseconds (23% faster) and 2
allocations (21% fewer bytes).
Some redundant checks are removed from UUID.UnmarshalJSON() method. The
"encoding/json".Unmarshaler interface specifies that implementations can
assume input is valid JSON content. This allows one to assume that (1)
input is not empty and (2) if index 0 is a quote, then the content is a
json string and the last index will contain a terminating quote. The
second point is not completely explicit in the documentation but it is
true in practice (and it is safe to assume -- errors will be caught).
