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(readchar): Increment it.
(unreadchar): Decrement it.
(read_multibyte): Decrement it.
(Vread_with_symbol_positions): New variable.
(Vread_symbol_positions_list): New variable.
(read_internal_start): New function, created from Fread and
Fread_from_string.  Handle Vread_symbol_positions_list and
Vread_with_symbol_positions.
(readevalloop, Fread, Fread_from_string): Use it.
(read1): Use readchar_count to add symbol positions to
Vread_symbol_positions_list if Vread_with_symbol_positions is
non-nil.
(syms_of_lread): DEFVAR_LISP and initialize them.