Redo some of the previous commit: add back the ability to use

Redo some of the previous commit: add back the ability to use
non-aliased/undeclared string literals since it might be useful to
those declaring %token-table.
* src/reader.c (check_and_convert_grammar): Undo changes in previous
commit: don't worry about complaints from symbols_pack.
* src/symtab.c (symbol_new, symbol_class_set,
symbol_check_alias_consistency): Undo changes in previous commit: count         each string literal as a new symbol and token, assign it a symbol
number, and don't complain about non-aliased string literals.
(symbols_pack): Since symbol_make_alias still does not decrement symbol
and token counts but does still set aliased tokens to the same number,
symbol_pack_processor now leaves empty slots in the symbols array.
Remove those slots.
* tests/regression.at (Undeclared string literal): Remove test case
added in previous commit since non-aliased string literals are allowed
again.
(Characters Escapes, Web2c Actions): Undo changes in previous commit:
remove unnecessary string literal declarations.
* tests/sets.at (Firsts): Likewise.