When changing a generated column's expression, a check for whether the
expression was the same as before was missing.  If the expression is
changed, storage engine may need to rebuild the column.
(A virtual column generally does not need rebuilding, but if is indexed,
the index will need rebuilding).

Two new utility functions to check that generation expressions are equal
are implemented: Field::gcol_expr_is_equal().

In fill_alter_inplace_info(), generate change commands for the storage
engine according to the following:

- It is possible a convert a base column to a generated column and vice
  versa as long as the generated column is stored. Set the handler flag
  ALTER_STORED_COLUMN_TYPE for this case.

- If both source and target column is generated and generation expression
  is modified, set handler flag ALTER_STORED_COLUMN_TYPE if column is
  stored and ALTER_VIRTUAL_COLUMN_TYPE if column is virtual.

Elaborate testing has been added:

- Check modification to and from generated column.

- Check modification of generated column with and without non-unique
  index and unique index.