Enforce stricter typing with PHP 7

[mnapoli]

Strict typing everywhere!

[holtkamp]

@mnapoli did you consider bumping the minimum required version to PHP 7.1 to allow nullable types?

This would enable to increase strictness even further, for example here:

    /**
     * @return bool|null
     */
    public function isLazy()
    {
        return $this->lazy;
    }

    /**
     * @return bool|null
     */
    public function isLazy(): ?bool
    {
        return $this->lazy;
    }

Combined with typed collections as described here, this makes a powerful combination 😄

[mnapoli]

@holtkamp, @Taluu, @avant1 I've opened #483 to discuss that.