Use an enum to set set_timeout_or_interval's is_interval field. Fixes #4059
remove comments now that we have more descriptive names
auto merge of #4064 : trevorriles/servo/isintervalenum, r=jdm

Created an `IsInterval` enum to improve readability and remove the need for `true // is_interval`

I'm still fairly new to rust. I briefly looked for a way to implement boolean comparisons of the enum but didn't figure out a way. 

Also I'm not attached to any of the names. Let me know what I can fix :)