This optimization is no longer necessary.  Thanks to Python 3.6's new
implementation of dicts, the memory footprint difference between slots
and dicts is now around 5%, which is no longer worth the complexity and
performance cost.

This saves 2% of the total installation time.

X-original-commit: c7f17770803744cbbb741fc11265ee914641aed7

This branch is the combination of several optimizations in the ORM:

* store field values once in the cache: the cache reflects more
faithfully the database, only fields that explicitly depend on the
context have an extra indirection in the cache;

* delay recomputations by default: use method `recompute` to explicitly
flush out pending recomputations;

* delay updates in method `write`: updates are stored in a data
structure that can be flushed efficiently to the database with method
`flush` (which also flush out recomputations);

* make method `modified` take advantage of inverse fields to inverse
dependencies;

* filter records by evaluating a domain on records in Python;

* a computed field with `readonly=False` behaves like a normal field
with an onchange method;

* computed fields are computed in superuser mode by default.

Work done by Toufik Ben Jaa, Raphael Collet, Denis Ledoux and Fabien
Pinckaers.

closes odoo/odoo#35659

Signed-off-by: Denis Ledoux <beledouxdenis@users.noreply.github.com>

Also remove method `self._check_selection_field_value`, and perform the check
in field method `convert_to_column`.

Let `create` and `write` round monetary field values before sending them to the
database. Pass the values to be written to `field.convert_to_column`, so that
the currency can be retrieved from the values, and the value be rounded.

This removal has led to a necessary refactoring:
- make the setup of field attributes extensible;
- make the instantiation of custom fields extensible;
- delegate model and field reflection to `ir.model` and `ir.model.fields`;
- move the implementation of sparse and serialized fields to module `base_sparse_field`;