Avoid partial functions Data.List.init and Data.List.last

[Bodigrim]

This is to comply with haskell/core-libraries-committee#292

[Bodigrim]

@hasufell just a gentle reminder.

[Bodigrim]

Sigh, I should have run benchmarks. hasTrailingPathSeparator regressed a lot, because getTrailingPathSeparator does not fuse well and we are left with a lot of overhead. And if we revert hasTrailingPathSeparator back and plug -Wno-x-partial, then there is no raison d'etre for other changes, because we need -Wno-x-partial anyway.

In general this performance issue could be worked out / worked around, but we have a very limited toolbox because of keeping both FilePath and OsPath implementations in the same place.

I think this should better be reverted and {-# OPTIONS_GHC -Wno-x-partial #-} added atop.

[hasufell]

I tried with cabal build --ghc-options='-Wall -Wextra -Wx-partial' and I don't get any warnings after reverting said commit.

GHC 9.14.0.20250819