optrep and rep parsers don't stop when parsing is partial

[d-plaindoux]

The parsers optrep and rep should stop when parsing consumes elements.

For instance the parser optrep (atom 'a' <&> atom 'b') recognises sequence abababab....
If this sequence is abac it should return an error instead of ('a','b') and the stream "ac". The backtrack is managed thanks to do_try parser only.

[d-plaindoux]

See #4