Implicitly return Parser instances

[Bikossor]

In some parsers and combinators (optional, lazy and failure) the Parser instance is returned explicitly. This differs from the usual implementation of using a concise body syntax (implicit return).

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#function_body

Before:

export const lazy = (parserThunk: () => Parser): Parser => {
  return new Parser(state => {
    return parserThunk().transformState(state);
  });
};

After:

export const lazy = (parserThunk: () => Parser): Parser =>
  new Parser(state => {
    return parserThunk().transformState(state);
  });