- sql_prepare.cc: Moving functions set_param_xxx() as
  methods to Item_param

- Replacing a pointer to a function Item_param::set_param_func
  to Type_handler based implementation:
  Item_param::value now derives from Type_handler_hybrid_field_type.
  Adding new virtual methods Type_handler::Item_param_setup_conversion()
  and Type_handler::Item_param_set_param_func()

- Moving declaration of some Item_param members  from "public:" to "private:"
  (CONVERSION_INFO, value, decimal_value)

- Adding a new method Item_param::set_limit_clause_param(),
  to share duplicate code, as well as to encapsulate
  Item_param::value.

- Adding Item_param::setup_conversion_string() and
  Item_param::setup_conversion_blob() to share
  the code for binding from a client value
  (mysql_stmt_bind_param), and for binding from
  an expression (Item).

- Removing two different functions set_param_str_or_null()
  and set_param_str(). Adding a common method Item_param::set_param_str().
  Item_param::m_empty_string_is_null, used by Item_param::set_param_str().

- Removing the call for setup_one_conversion_function() from
  insert_params_from_actual_params_with_log(). It's not needed,
  because the call for ps_param->save_in_param() makes sure
  to initialized all data type dependent members properly,
  by calling setup_conversion_string() from
  Type_handler_string_result::Item_param_set_from_value()
  and by calling setup_conversion_blob() from
  Type_handler_blob_common::Item_param_set_from_value()

- Cleanup: removing multiplication to MY_CHARSET_BIN_MB_MAXLEN
  in a few places. It's 1 anyway, and will never change.