Compiler improvements

Catch-all for upcoming compiler improvements, aiming for 1.7.

• backref and lastline currently cause a full scope to be created for a given method. This slows any code that calls innocuous methods like []. However, only a NoVars scope should be necessary, to just have room for the backref/lastline slots.

• backref and lastline could be shunted to some other stack or thread-local structure, instead of requiring a scope. This would greatly reduce the impact of methods as described above (no scope at all would be neccessary; worst case, an int would need to be incremented/decremented in ThreadContext.

• Constant accesses currently require a dummy scope to be pushed/popped for any methods that contain them. This could be eliminated by making sure the method has access to its own StaticScope, which is the only additional structure needed to lookup constants. Combined with invokedynamic constant lookup, this would make constant access essentially free.