Solve holes in nats in typeEqRigid

[acl-cqc]

https://github.com/CQCL/brat/blob/e6e37a9bb264957679cbcafd4c9b96583b42a1e0/brat/Brat/Checker/SolveHoles.hs#L140-L141

We are not even trying to solve holes, this is probably what's hitting us with examples such as

unfull(n :: #, v :: Vec(Bool, full(n))) -> Nat
unfull(n, _) = n

two :: Nat
two = unfull(!, [false, false, false])

with error message (on branch just-nat-solving (#70) or inference-wip/fork)

Expected vector of length (2^(VPar In check_defs_1_two_1_Eval 0) - 1)
from the type:  Vec(Bool, (2^(VPar In check_defs_1_two_1_Eval 0) - 1))
but got vector: [false(()),false(()),false(())]
of length (> 0)