[FIX] fields: fix `_column.new()` by relying on `to_field_args()`

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.

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