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(symbol_block_pinned): New var.
(Fmake_symbol): Initialize `pinned'.
(purecopy): New function, extracted from Fpurecopy.  Mark symbols as
pinned and signal an error for un-purifiable objects.
(pure_cons): Use it.
(Fpurecopy): Use it, except for objects that can't be purified.
(mark_pinned_symbols): New function.
(Fgarbage_collect): Use it.
(gc_sweep): Remove hack made unnecessary.
* src/lisp.h (struct Lisp_Symbol): New bitfield `pinned'.