Way too many global variables

[CelticMinstrel]

There are tons and tons of global variables tracking all manner of state and it's pretty confusing. We should reduce the number of globals as much as possible – even just gathering related individual globals into a global struct would be a huge improvement to how things are now.

The fact that some targets require spurious declarations just to compile is also a problem. We should avoid declaring extern globals in headers that are included in more than one target.