The code passing positions in the query to constructors of
Rewritable_query_parameter descendants (e.g. Item_splocal)
was not reliable. It used various Lex_input_stream methods:
- get_tok_start()
- get_tok_start_prev()
- get_tok_end()
- get_ptr()
to find positions of the recently scanned tokens.

The challenge was mostly to choose between get_tok_start()
and get_tok_start_prev(), taking into account to the current
grammar (depending if lookahead takes place before
or after we read the positions in every particular rule).

But this approach did not work at all in combination
with token contractions, when MYSQLlex() translates
two tokens into one token ID, for example:
   WITH ROLLUP -> WITH_ROLLUP_SYM

As a result, the tokenizer is already one more token ahead.
So in query fragment:

  "GROUP BY d, spvar WITH ROLLUP"

get_tok_start() points to "ROLLUP".
get_tok_start_prev() points to "WITH".

As a result, it was "WITH" who was erroneously replaced
to NAME_CONST() instead of "spvar".

This patch modifies the code to do it a different way.

Changes:

1. For keywords and identifiers, the tokenizer now
returns LEX_CTRING pointing directly to the query
fragment. So query positions are now just available using:
- $1.str           - for the beginning of a token
- $1.str+$1.length - for the end of a token

2. Identifiers are not allocated on the THD memory root
in the tokenizer any more. Allocation is now done
on later stages, in methods like LEX::create_item_ident().

3. Two LEX_CSTRING based structures were added:
- Lex_ident_cli_st - used to store the "client side"
  identifier representation, pointing to the
  query fragment. Note, these identifiers
  are encoded in @@character_set_client
  and can have broken byte sequences.

- Lex_ident_sys_st - used to store the "server side"
  identifier representation, pointing to the
  THD allocated memory. This representation
  guarantees that the identifier was checked
  for being well-formed, and is encoded in utf8.

4. To distinguish between two identifier types
   in the grammar, two Bison types were added:
   <ident_cli> and <ident_sys>

5. All non-reserved keywords were marked as
   being of the type <ident_cli>.
   All reserved keywords are still of the type NONE.

6. All curly brackets in rules collecting
   non-reserved keywords into non-terminal
   symbols were removed, e.g.:

   Was:

       keyword_sp_data_type:
         BIT_SYM           {}
       | BOOLEAN_SYM       {}

   Now:

       keyword_sp_data_type:
         BIT_SYM
       | BOOLEAN_SYM

  This is important NOT to have brackets here!!!!
  This is needed to make sure that the underlying
  Lex_ident_cli_ststructure correctly passes up to
  the calling rule.

6. The code to scan identifiers and keywords
  was moved from lex_one_token() into new
  Lex_input_stream methods:

   scan_ident_sysvar()
   scan_ident_start()
   scan_ident_middle()
   scan_ident_delimited()

  This was done to:
  - get rid of enormous amount of references to &yylval->lex_str
  - and remove a lot of references like lip->xxx

7. The allocating functionality which puts identifiers on the
   THD memory root now resides in methods of Lex_ident_sys_st,
   and in THD::to_ident_sys_alloc().
   get_quoted_token() was removed.

8. Cleanup: check_simple_select() was moved as a method to LEX.

9. Cleanup: Some more functionality was moved from *.yy
   to new methods were added to LEX:
     make_item_colon_ident_ident()
     make_item_func_call_generic()
     create_item_qualified_asterisk()