The implementation was based on the ill-defined method `same_parameters()` that
compares arguments based on a heuristic. Instead, we now create a new column
and check whether it is equivalent to `self` by comparing the arguments
returned by `to_field_args()`. If that is the case, `self` is reused instead
of the new column.
The code refactoring also fixes the column reuse which was broken by the
introduction of the parameter `compute` in commit 9333c62. Indeed, with that
parameter, `same_parameters()` always returned False, since old-api columns do
not have that parameter by default. The parameter has been renamed to
`_computed_field`, and is no longer passed for creating columns.
5ec8596f67