Add #n=object, #n#, and #:symbol constructs to reader.

Add #n=object, #n#, and #:symbol constructs to reader.
(readevalloop, read, Fread_from_string): Empty list of read objects before
read0 call.
(read1): New variable `uninterned_symbol', which controls how to make
symbols.  Support #:, #n=object and #n#.
(make_symbol): New function, used in read1 to make uninterned symbols
(Fintern): Set `obarray' field of interned symbols.
(init_obarray): Explicit set `obarray' field of symbol `nil'.
(syms_of_lread): staticpro read_objects, the list of read objects.