Restore PHP typehints

[stevegrunwell]

For version 0.1.0 of the library, we're going to support PHP 5.6 as this is the current minimum PHP version for WordPress.

Eventually, WordPress will drop PHP 5.6 (to be honest, that day can't come soon enough) and then we'll be able to safely drop it from our test matrix and update the code to use PHP 7.x-specific features like scalar typehints.

This ticket is here to make sure that happens.