The new convention is that any parsing function returning `Err(())`
may or may not have consumed any number of tokens from the input.

Therefore, any the caller wishes to recover and try parsing something else,
it should call `parser.reset(position)` where `position` is the result of
`parser.position()` that was saved beforehand.

The Parser::try helper does this for you by taking a closure returns Result,
and resetting the position if the result is Err.