Important: this changes the semantics of the write barrier!
Carefully read the big comment block in lj_obj.h
This helps HREFK key slot specialization and allows safely hoisting
HREF/HREFK across GC steps, too (fix for a barely reproducible bug).
Dead keys are only removed during a table resize (as before).
Add 64 bit lightuserdata handling. Keep the tagged 64 bit value.
Allocate/save/restore 64 bit spill slots for 64 bit lightuserdata.
Fix code generation for 64 bit loads/stores/moves/compares.
Fix code generation for stack pointer adjustments.
Add fixed spill slot definitions for x64. Reduce reserved spill slots.
Disable STRREF + ADD fusion in 64 bit mode (avoid negative 32 bit ofs).
Compile math.sinh(), math.cosh(), math.tanh() and math.random().
Compile various io.*() functions.
Drive the GC forward on string allocations in the parser.
Improve KNUM fuse vs. load heuristics.
Add abstract C call handling to IR.
