Add support for single-expression methods

[thekid]

This PR adds syntactic support for single-expression methods

class Person {
  public function __construct(private string $name) { }

  // Equivalent of public function name() { return $this->name; }
  public function name() => $this->name;
}

See:

• https://wiki.php.net/rfc/single-expression-functions
• https://github.com/xp-lang/php-compact-methods (same but uses fn)
• feat: single expression functions php/php-src#17677

[thekid]

RFC has been declined

[thekid]

This would be consistent with the abbreviated property hooks syntax:

class User {
  public function __construct(private string $first, private string $last) {}
 
  // Long form
  public string $fullName {
    get { 
      return $this->first.' '.$this->last;
    }
  }

  // Short form
  public string $fullName {
    get => $this->first.' '.$this->last;
  }

  // Long form
  public string $username {
    set(string $value) {
      $this->username= strtolower($value);
    }
  }
 
  // Short form
  public string $username {
    set => strtolower($value);
  }
}

...which sets => [exor] as an equivalent of { return [expr]; }.

[thekid]

This is inconsistent with how short and long forms of property accessors are parsed - they make use of the Block class:

• body for => [expr]: Expr
• body for { [statement]; }: Block([Statement])