Don't let fuzz targets import arbitrary directly (#4806)

The version of the `arbitrary` crate used in fuzz targets needs to be
the same as the version used in `libfuzzer-sys`. That's why the latter
crate re-exports the former.

But we need to make sure to consistently use the re-exported version.
That's most easily done if that's the only version we have available.
However, `fuzz/Cargo.toml` declared a direct dependency on `arbitrary`,
making it available for import, and leading to that version being used
in a couple places.

There were two copies of `arbitrary` built before, even though they were
the same version: one with the `derive` feature turned on, through the
direct dependency, and one with it turned off when imported through
`libfuzzer-sys`. So I haven't specifically tested this but fuzzer builds
might be slightly faster now.

I have not removed the build-dep on `arbitrary`, because `build.rs` is
not invoked by libFuzzer and so it doesn't matter what version of
`arbitrary` it uses.

Our other crates, like `cranelift-fuzzgen` and `wasmtime-fuzzing`, can
still accidentally use a different version of `arbitrary` than the fuzz
targets which rely on them. This commit only fixes the direct cases
within `fuzz/**`.

fuzz/fuzz_targets/cranelift-fuzzgen.rs

@@ -15,7 +15,7 @@ use cranelift_interpreter::interpreter::{
 };
 use cranelift_interpreter::step::ControlFlow;
 use cranelift_interpreter::step::CraneliftTrap;
-use smallvec::{smallvec, SmallVec};
+use smallvec::smallvec;
 
 const INTERPRETER_FUEL: u64 = 4096;