innobase_rename_column(): New function, for renaming a column in both
the InnoDB data dictionary and the data dictionary cache. If the column
name becomes longer, allocate a new table->col_names from table->heap.
Otherwise, overwrite the existing table->col_names.

ha_innobase::commit_inplace_alter_table(): Invoke innobase_rename_column()
when needed.

The performance could be improved by doing the rename in one step.
Currently, we perform the operation on every renamed column separately.
A bigger performance improvement would be gained if the InnoDB data
dictionary were eliminated altogether and replaced by something that
plays well with InnoDB crash recovery.