grammar: accept annonymous label

grammar: accept annonymous label

Often, a single label is needed, for aborting some neested loops for
instance. But the programmer was forced to still gives a name to a
label. This yielded to crasy or useless names,

The proposal is to allow anonymous label.

Eg. in the following, the break breaks the x loop
~~~
for x in xs do
  for y in ys do
    stuff
    if something then break label
  end
  stuff
end label
~~~

Note: by the way, valued breaks and continues are removed.

Signed-off-by: Jean Privat <jean@pryen.org>