Moving a few static functions in sql_lex.cc to new methods in Lex_input_stream

Reasoning:
- Shorter and clearer code
- Better encapsulation
  (a fair number of Lex_input_stream methods and members were
   moved to the private section)

New methods:

  int lex_token(union YYSTYPE *yylval, THD *thd);
  bool consume_comment(int remaining_recursions_permitted);
  int lex_one_token(union YYSTYPE *yylval, THD *thd);
  int find_keyword(Lex_ident_cli_st *str, uint len, bool function);
  LEX_CSTRING get_token(uint skip, uint length);

Additional changes:

- Removing Lex_input_stream::yylval.
  In the original code it was just an alias
  for the "yylval" passed to lex_one_token().
  This coding style is bug prone and is hard to follow.
  In the new reduction "yylval" (or its components) is passed to
  the affected methods as a parameter.
- Moving the code in sql_lex.h up and down between "private" and "public"
  sections (sorry if this made the diff somewhat harder to read)

sql/sql_class.h

@@ -4250,7 +4250,7 @@ public:
     Lex_input_stream *lip= &m_parser_state->m_lip;
     if (!yytext)
     {
-      if (lip->lookahead_token >= 0)
+      if (lip->has_lookahead())
         yytext= lip->get_tok_start_prev();
       else
         yytext= lip->get_tok_start();