[nmolinamounier]

Motivation for this change

Fixes #758 by adding extra display arguments to topologies, in the same way as orders in math-comp.

A new type fdisp_t is added in filter.v, and every topology is associated with an instance of fdisp_t, which can be used in notations for proper display.

Since it is common to define specific topologies in terms of more generic ones, with_fdisp d' T has the same topology as T, but with display d'.

By default, displays aren't shown on common topology functions (nbhs, open, etc.). When needed, the module ExplicitNbhsDisplays can be imported to make them explicit.

This PR requires changing every single definition involving a filtered type to add a display argument, hence its very large footprint.

About order topologies : right now, order topologies have two arguments for topology and order displays respectively. I tried merging them into a record, but this doesn't seem to play well with canonical structure inference ; we should discuss this.

Checklist

• appended entries in the relevant sections of CHANGELOG_UNRELEASED.md
• Fix the current state of order topologies (?)
• Bikeshed notations and naming

• added corresponding documentation in the headers

Reference: How to document

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