Convert simple arrow function body to expression

[ashsearle]

Eliminate unnecessary return keyword by converting arrow function block statements to expressions where possible.

Example:

const foo = () => {
  return (1, 2);
};
const bar = () => {
  return [];
};

// becomes
const foo = () => (1, 2);
const bar = () => [];

[boopathi]

We are in a BlockStatement - is this check ever falsey ? A BlockStatement's body is always an array.