Get rid of LEX::length and LEX::dec

Passing data in sql_yacc.yy through LEX members is hard to follow and is not recursively safe.
We'll get rid of LEX::length and LEX::dec and add a structure instead:

struct Lex_length_and_dec_st

{

  const char *length;

  const char *dec;

};

so it can be passed through the syntax rules in sql_yacc.yy as $$, $1, $2, etc.
This will help, for example, to change:

field_spec:

  field_ident { /* new Create_field is allocated here */}

  field_type { Lex->set_last_field_type($3); }

  field_def { ... }

  ;

into:

field_spec:

  field_ident field_type { /* new Create field is allocated here*/ }

  field_def { ... }

So we'll already know data type, length and dec when we do "new Create_field".
This will allow to call a method in Type_handler, to create a data-type specific "Create_field" replacement, instead of using the generic universal Create_field.