Adjust strictness of from{Asc,Desc}List* for maps

[meooow25]

Make the functions strict in the first value of the run of equal keys. This makes the strictness match that of the fromList functions.

Fixes #473.

No impact on benchmarks, on GHC 9.6.3

Map:
  fromAscList: OK
    65.2 μs ± 5.4 μs, 686 KB allocated,  21 KB copied, 9.0 MB peak memory,       same as baseline

IntMap:
  fromAscList: OK
    46.3 μs ± 2.7 μs, 223 KB allocated, 6.1 KB copied,  10 MB peak memory,       same as baseline