so that all integer fields can be packed into 64 bits.  (Bug #20877)

dtype_t: Change the type of all bit-fields to unsigned.

dict_table_get_nth_col(), dict_table_get_sys_col_noninline(),
dict_table_get_sys_col(), dict_field_get_col(): Return const
dict_col_t*, so that changes to dict_col_t can be detected more
easily.  Add const to many dict_col_t* declarations.

dict_index_get_nth_type(): Replace with dict_index_get_nth_col().

dict_col_get_type(): Replace with dict_col_copy_type().

dict_col_get_min_size(), dict_col_get_max_size(), dict_col_get_fixed_size(),
dict_col_get_sql_null_size(): New functions.

dtype_get_at_most_n_mbchars(): Replace the parameter dtype
with the parameters prtype, mbminlen, mbmaxlen.

dtype_get_pad_char(), cmp_data_data(), cmp_data_data_slow(),
cmp_whole_field(): Replace the dtype_t* parameter with the ulint
parameters mtype, prtype.

dtype_copy(): Add a const qualifier to type2 (the one being copied from).

dtype_set_mblen(): Replaced with dtype_get_mblen().

dtype_get_fixed_size_low(), dtype_get_min_size_low(),
dtype_get_fixed_max_low(): Replace dtype_get_fixed_size(),
dtype_get_min_size(), and dtype_get_max_size().  These are used by the
dict_col_get_{fixed,min,max}_size() functions.

cmp_types_are_equal(): Replace with cmp_cols_are_equal().

dict_table_get_col_name(): Add a const qualifier parameter to the
parameter "table".

dtype_binary, dtype_binary_val: Remove.

dtype_is_fixed_size(): Remove.