Is there some sort of an iRule "include" statement?

Unfortunately, no

You can simulate reusable code by separating it into different iRules. You can have multiple iRules of the same event and order them using priority. Then whenever you need to update your iRule, all the virtual's using it will update themselves when you save the changes.

when HTTP_REQUEST priority 300 {
    reusable code
}
when HTTP_REQUEST priority 500 {
    normal code
} 

As of 11.3 you can you functions as I recall. Functions are globally available, so you can have one iRule where functions are defined.

you know irule procedure is available in 11.4.0, don't you? :-)

Procedures/Subroutines for iRules have arrived in 11.4.0!

Procs are indeed how you would do this, and are new and awesome as of v11.4:

There will be lots more information and examples in coming weeks, so keep an eye out. ;)