Some more Library.php cleanup

Continuing on the slow death of lib.php, this starts the process of breaking out some of the more basic methods into helper groupings.

This is intended to be a 'phase 1' or sorts to ease refactoring into proper services in cases where it's possible, although in attempting to set up a service right from the start, I've found some methods (namely string methods) are used before $app has been initialized.

This PR currently covers the following helpers:
 - Input
 - String
 - Arr
 - Html

The following are other possible groups remaining, but may need some extra care/discussion:
 - Path
 - Redirect
 - File
 - Template
 - Serializer