row_metadata_to_tuple(): Convert a metadata record to a data tuple,
based on the new info_bits of the metadata record.

btr_cur_pessimistic_update(): Invoke row_metadata_to_tuple() if needed.

dict_index_t::instant_add_field(): Append fields at the end, if any.
The fields for dropped columns are preserved.

dict_table_t::instant_column(): Renamed from instant_add_column().
Add the parameter col_map so that columns can be reordered.
FIXME: Also support ADD/DROP/reorder/rename of VIRTUAL COLUMN.

ha_innobase_inplace_ctx::prepare_instant(): Preserve any instantly
dropped columns in instant_table and its first index.
FIXME: Support instant ALTER TABLE even if hidden FTS_DOC_ID exists.

innobase_instant_try(): Simplify the logic for detecting dropped
or reordered columns. If any drop or reorder is needed, construct
an update vector with the metadata blob.

dtype_t::metadata_blob_init(): Initialize the metadata BLOB data type.

dict_table_t::find(): Find an old column based on a new column number.

rec_convert_dtuple_to_metadata_comp(): Convert an alter metadata tuple
into a record.

row_rec_to_index_entry_impl(): Add the template parameter mblob
and the optional parameter info_bits for specifying the desired new
info bits. For the metadata tuple, allow conversion between the original
format (ADD COLUMN only) and the generic format (with hidden BLOB).

row_upd_index_replace_metadata(): Apply an update vector to an
alter_metadata tuple.