feat(fun_prop): allow fun_prop to call discharger on ModelWithCorners

[lecopivo]

Currently, a discharger in fun_prop is allowed to be run only on Prop-valued hypothesis.
This PR allows to execute dischargers also on ModelWithCorners. This will enable fun_prop to support ContMDiff properly: lemmas like ContMDiff.comp require inferring the model with corners on the intermediate space. This PR is thus required to properly support manifolds in fun_prop; a future PR will implement this.

In the future, we might want to allow the discharger to execute on other Type-valued hypothesis.
In this case, we could create an environment extension to register such types.
However, right now we could not think of any other use cases --- therefore, we hard-coded ModelWithCorners.

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[github-actions]

PR summary 1b6d405437

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ Dummy
+ ModelWithCorners
+ ModelWithCorners.prod
+ apply
+ comp
+ const
+ fst
+ id
+ instance : Dummy A := ⟨⟩
+ mdiff_add
+ mdiff_mul
+ pi
+ prod_mk
+ snd

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.

[grunweg]

As we discussed in person: thanks a lot, this is great! Please add the test you created locally, once your local build has worked. (I should also make a prototype branch to hack in ContMDiff support; that's one me.)

[grunweg]

Only minor style comments. (I like cleanly written comments.)

[grunweg]

Oh, it seems I just force-pushed your work away... let me fix that! (edit: done)