Askrene single path solver

[Lagrang3]

With this PR I want to include an internal API to askrene to compute optimally reliable
single path payments. It uses the same costs functions as the minimum cost flow solver
but with the additional constraint that only a single path solution will be considered.
In practice this is solved with the execution of Dijkstra best path algorithm therefore it is likely
to produce orders of magnitude (~x100?) faster run times.

This option can be activated by calling getroutes with the layer auto.no_mpp_support
or with the developer flag --dev_algorithm=single-path.

We used to compute single path routes by first doing a MCF computation and then
discarding all but one path which is computationally expensive and not even optimal.

We may use this single path computation with small payment amounts or situations for which MPP
is not supported or if time to solution is a concern.

Depends on #8163 and #7932.

TODO:

• documentation
• tests
• benchmarks

[Lagrang3]

I benchmarked the single path solver in comparison to the MPP default solver
on the compressed gossmap "./tests/data/gossip-store-2024-09-22.compressed"
by simulating a payment from node 3301 to the first ordered 500* nodes,
the amount is 1M sat and maximum fee set to 1k sat.

The success rate for MPP is 82% while single path is 77%.

I wanted to test the routing to the first 1000 nodes but askrene crashed while trying to reach node 785, this bug can also be reproduced in the master branch. I am digging into it.
The bug is a bad memory access due to miss interpretation of operator precedence: *flow[i] vs (*flow)[i].
It is fixed in the preceding PR #8163 where the bug was introduced.

[rustyrussell]

Minor feedback only... I'm reworking the predecessor PR, so this will need a rebase too..

[Lagrang3]

Rebased to fix conflicts with previous changes on #8163

[Lagrang3]

Wow, this is weird. I didn't touch renepay code at all and this test decided to randomly fail.

=========================== short test summary info ============================
FAILED tests/test_renepay.py::test_hardmpp2 - pyln.client.lightning.RpcError: RPC call failed: method: renepay, payload: {'invstring': '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'}, error: {'code': 205, 'message': "minflow couldn't find a feasible flow: failed to find a feasible flow: find_admissible_path failed"}
============= 1 failed, 78 passed, 8 skipped in 2093.60s (0:34:53) =============

https://github.com/ElementsProject/lightning/actions/runs/16217132309/job/45790820690?pr=8299#step:8:8140

UPD: this must have been a glitch in the CI.
Otherwise this PR is ready for merging.