_upb_Arena_FastMalloc() has been directly inlined
Arena::allocator() has been removed
mem_block is now _upb_MemBlock
some calls to upb_Arena_Alloc() (which is going away) have been changed to use upb_Arena_Malloc() instead
PiperOrigin-RevId: 489874377
Lots of changes but it's all just moving things around.
Backward-compatible stub #include's have been provided for now.
upb_Arena/upb_Status have been split out from upb/upb.?
upb_Array/upb_Map/upb_MessageValue have been split out from upb/collections.?
upb_ExtensionRegistry has been split out from upb/msg.?
upb/decode_internal.h is now upb/internal/decode.h
upb/mini_table_accessors_internal.h is now upb/internal/mini_table_accessors.h
upb/table_internal.h is now upb/internal/table.h
upb/upb_internal.h is now upb/internal/upb.h
PiperOrigin-RevId: 456297617