lexer: Change the functions for retrieving token strings.

lexer: Change the functions for retrieving token strings.

Until now, lex_tokid() has been for T_ID tokens only and lex_tokstr() has
been for T_ID and T_STRING tokens.  For T_ID tokens, lex_tokid() and
lex_tokstr() had slightly different semantics.

This doesn't entirely make sense, and these particular functions are not
the ones wanted most by clients, so this commit removes these functions
in favor of lex_tokcstr() and lex_tokss(), which are both applicable
to both T_ID and T_STRING tokens, with the same semantics in each case.

These functions are also easier for the upcoming reimplementation of the
lexer.