The `libfuzzer-sys` update in #5068 included some changes to the
`fuzz_target!` macro which caused a bare `run` function to be shadowed
by the macro-defined `run` function (changed in
rust-fuzz/libfuzzer#95) which meant that some of our fuzz targets were
infinite looping or stack overflowing as the same function was called
indefinitely. This renames the top-level `run` function to something
else in the meantime.