Fix some memory leaks

[jkriegshauser]

• Fixed: Historically, the GPIO command was leaked because of a crash, though this is no longer happening in local testing. If it recurs, we should debug the crash (which may be easier with ASan support now)
• Fixed: Leaking dict (and possible leaking channel layouts) in SDL code
• Fixed: ScheduledItem leaks in the Scheduler
• Changed: ScheduledItem and vectors in the Scheduler are no longer separate heap allocations, which should make management easier, reduce heap usage and fragmentation, etc.

[jkriegshauser]

@dkulp review please? I just rebased to fix conflicts

[dkulp]

Think I got everything resolved and merged.... Removed the typedef as I prefer to see the std container types.

[jkriegshauser]

@dkulp thanks! I don't mind making tweaks too if different things are preferred :D