This contains all the accessor functions and macros out of lfs.h.
Add an include of lfs_accessors.h after all uses of lfs.h... except
for code that wants to define its own struct lfs-alike that the
accessors are supposed to play along with. For these, set STRUCT_LFS
and include lfs_accessors.h after the necessary structure has been
defined, so that lfs_accessors.h can emit functions in terms of it.
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