Don't allow an undeclared string literal, but allow a string literal to

Don't allow an undeclared string literal, but allow a string literal to
be used before its declaration.
* src/reader.c (check_and_convert_grammar): Don't invoke packgram if
symbols_pack complained.
* src/symtab.c (symbol_new): Don't count a string literal as a new
symbol.
(symbol_class_set): Don't count a string literal as a new token, and
don't assign it a symbol number since symbol_make_alias does that.
(symbol_make_alias): It's not necessary to decrement the symbol and
token counts anymore.  Don't assume that an alias declaration occurs
before any uses of the identifier or string, and thus don't assert that
one of them has the highest symbol number so far.
(symbol_check_alias_consistency): Complain if there's a string literal
that wasn't declared as an alias.
(symbols_pack): Bail if symbol_check_alias_consistency failed since
symbol_pack asserts that every token has been assigned a symbol number
although undeclared string literals have not.
* tests/regression.at (String alias declared after use, Undeclared
string literal): New test case.
(Characters Escapes, Web2c Actions): Declare string literals as
aliases.
* tests/sets.at (Firsts): Likewise.