Add 'label' syntax construction

Add 'label' syntax construction

A label can be used in 'break' and 'continue' statements
A label can be declared in 'for', 'while', 'do' and closure constructions.

Note that in grouped closures, only the last closure is labeled but
it applies to all closures of the group:

 foo with do
   a
 with do
   b
 end label xxx

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