tryParseUnsigned*

[cpovirk]

Another discovery of our @CheckReturnValue additions is that people (~8 in Google-internal code) write code like this:

try {
  UnsignedLongs.parseUnsignedLong(foo);
  return true;
} catch (NumberFormatException e) {
  return false;
}

When parseUnsignedLong gets its @CheckReturnValue annotation, this requires a suppression.

If we provided tryParseUnsignedLong and friends, they could use that instead.