Non-functional code tidying

Hi,

These changes are all, I believe, entirely non-functional (i.e. they don't change any Phockito behaviour), they just satisfy static analysis tools (such as provided by PHPStorm / IntelliJ). Many of the changes are just tweaks to the type hinting to help avoid false positives.

This may not seem hugely important, but some people (myself included) like to keep static analysis warnings to a minimum (to make it easier to spot genuine problems reported by said analysis); hopefully these changes will help them (me) in that endeavour.

Thanks,
Rowan