Support core.v0.15.1

[gfngfn]

$ opam pin add sesterl . failed with:

#=== ERROR while compiling sesterl.0.2.1 ======================================#
# context     2.1.4 | macos/arm64 | ocaml-base-compiler.4.14.1 | pinned(git+file:///(omitted)/Sesterl#master)
# path        ~/.opam/4.14.1/.opam-switch/build/sesterl.0.2.1
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p sesterl -j 7 @install
# exit-code   1
(omitted)
# File "src/myUtil.ml", line 92, characters 59-71:
# 92 |   let f = if canonicalize then Core.Filename.realpath else (fun s -> s) in
#                                                                 ^^^^^^^^^^^^
# Error: This expression should not be a function, the expected type is
#        [ `Use_Filename_unix ]