feat: generalize StateT instances to work for Functor

[eric-wieser]

This PR allows the use of StateT σ m when only Functor m is available.

This also results in a possibly more efficient StateT.map, since it now uses the map from the underlying functor rather than discarding it and rebuilding with bind.

[eric-wieser]

changelog-library

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase f1737737f0699ac8417605dc0148f079b5c8dd53 --onto c4e5f575121d508e02c82edd603f3c2808bdb5bc. You can force Mathlib CI using the force-mathlib-ci label. (2025-09-02 12:12:33)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase f1737737f0699ac8417605dc0148f079b5c8dd53 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-02 12:12:34)

[eric-wieser]

The failures here look a little annoying; I think that #11470 (and some cleanup on top of it) will help