parser: oneline assignment for 'return' and 'continue'

parser: oneline assignment for 'return' and 'continue'

Example
  fun foo: Int = 5
is equivalent to
  fun foo: Int do return 5
and
  foo !bar = 5
is equivalent to
  foo !bar do continue 5

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