avm2: Class refactor

[Lord-McSweeney]

This refactors classes to match avmplus, which simplifies a lot of Class-related stuff and improves the optimizer.

Notable changes:

• There are now two Classes for every class. One holds the instance info, while the other holds the class info. Each of them holds a link to the other.
• The core class weave is simplified
  • init_instance_vtable is now called from within ClassObject::from_class_partial
  • The global global class is no longer special
• ClassObject now delegates to Class for the instance allocator
• ClassObject no longer stores its class vtable on itself
• TraitKind::Class slots on vtables are now properly typed (this should help some optimizations)
• The ClassObject -> symbol registry is now a Class -> symbol registry; hopefully this can't break anything
• ScriptObjectData now stores a Class instead of a ClassObject to represent its constructor
• describeType is simplified
• Classless objects no longer exist; instance_class isn't Optional anymore
• void no longer has a ClassObject

Box2d optimization stats:

FindPropStrict -> GetScriptGlobals : 62.18%
FindPropStrict -> GetOuterScope : 7.76%
FindPropStrict -> GetScopeObject : 8.24%

FindProperty -> GetOuterScope : 2.83%
FindProperty -> GetScopeObject : 94.34%

InitProperty -> SetSlot : 38.74% (-2.19%)
InitProperty -> SetSlotNoCoerce : 46.7% (+2.19%)

SetProperty -> SetSlot : 23.95% (-1.57%)
SetProperty -> SetSlotNoCoerce : 44.76% (+2.10%)
SetProperty -> CallMethod : 1.57%

GetProperty -> GetSlot : 73.69% (+6.38%)
GetProperty -> CallMethod : 0.34%

CallProperty -> CallMethod : 70.97% (+29.84%)
CallPropVoid -> CallMethod : 75.38% (+1.14%)

Coerce -> Nop : 51.71% (+1.93%)

CoerceD -> Nop : 81.34%

CoerceB -> Nop : 10.81% (+2.70%)

CoerceU -> Nop : 50%

CoerceI -> Nop : 33%

ReturnValue -> ReturnValueNoCoerce : 71.56%