Improve return statement

[epatrizio]

Many questions came up during return implementation about adding some syntax strong controls.
for example, return options only in a specific context (in a function blocks for example)

Where to implement this stuff : in menhir parser.mly ?
What is the best practice ?

béta version :
67691ab

Spec :
https://www.lua.org/manual/5.4/manual.html#3.3.4

[epatrizio]

Same issue with break statement :
break must be inside a loop (for, while, repeat)

[epatrizio]

Idea for break statement implementation in parser.mly :

let block_no_break :=
  | stats = list(stat); retstat = option(retstat); {
    if List.for_all (fun stat -> stat <> Sbreak) stats then 
      match retstat with
      | None -> stats
      | Some retstat -> stats @ [ retstat ]
    else error "break statement outside a loop"
}

But it seems a bit tricky to implement.
For example, if true then break end isn't possible in chunk but ok in a for block.