Modality.Value.Const should behave like a product

[riaqn]

Based on #3949

This PR refactors the interface of Modality.Value.Const to make it behave like a product of axes (that you can project and update specific axis) instead of a sequence of atoms (that you compose).

This allows other incoming PRs that do the following:

• remove [@no_mutable_implied_modalities], since we can spell out modalities to override those
• fix the behavior of default modalities, so the overriding is per-axis.
• warnings for specifying the same modality axis multiple times in a single modality expression.
• warnings for superfluous overriding (e.g., overriding portable with portable)

Review

@dkalinichenko-js , could you look at the changes in typemode.ml and tests. In particular, could you check that the implication and the print back is still as expected. Feel free to add more tests.