dtype_t: Remove unused field "prec", which was supposed to be used for
the precision of decimal columns in stand-alone InnoDB.

dtype_get_prec(): Remove.

dtype_set(), dict_mem_table_add_col(): Remove parameter "prec".

dtype_t: Turn all fields (mtype, prtype, len, mbminlen, mbmaxlen, len)
into bit-fields.

dict_table_t, dict_index_t, dict_tree_t: Omit magic_n from non-debug builds.

dict_col_t: Turn ind, clust_pos, and ord_part into bit-fields.

Replace the default clust_pos value ULINT_UNDEFINED with
REC_MAX_N_FIELDS and replace all references to clust_pos with calls to
the accessor function dict_col_get_clust_pos().

dict_field_t: Turn prefix_len and fixed_len into bit-fields.

dict_tree_t: Remove pad[64].

dict_table_t: Turn the fields ibd_file_missing, tablespace_discarded,
cached, flags, stat_initialized, and autoinc_inited into bit-fields.
Remove does_not_fit_in_memory from non-debug builds.

dict_index_t: Turn the fields trx_id_offset, n_user_defined_cols,
n_uniq, n_def, n_fields, n_nullable, and cached into bit-fields.

dict_foreign_struct: Turn n_fields and type into bit-fields.

rw_lock_t: Turn cline, last_s_line, and last_x_line into bit-fields.
Omit level unless #defined UNIV_SYNC_DEBUG.

Move REC_MAX_N_FIELDS (and REC_MAX_HEAP_NO and REC_MAX_N_OWNED)
from rem0rec.c to rem0types.h, as they are needed in dict0dict.ic.
dict_col_get_clust_pos(): Map REC_MAX_N_FIELDS to ULINT_UNDEFINED.