HexStr can be called with anything that bas begin() and end() functions,
so clean up the redundant calls.

(context: I tried to convert HexStr to use span, but this turns out to
be somewhat more involved than I thought, because of the limitation to
pre-c++17 Span lacking iterator-based constructor) . This commit is a
first step which stands on its own though)

ninja all check-all

ninja bitcoin-fuzzers
./test/fuzz/test_runner.py <path_to_corpus>