Add differential fuzzing against wasmi (a Wasm interpreter).

This PR adds a new fuzz target, `differential_wasmi`, that runs a
Cranelift-based Wasm backend alongside a simple third-party Wasm
interpeter crate (`wasmi`).  The fuzzing runs the first function in a
given module to completion on each side, and then diffs the return value
and linear memory contents.

This strategy should provide end-to-end coverage including both the Wasm
translation to CLIF (which has seen some subtle and scary bugs at
times), the lowering from CLIF to VCode, the register allocation, and
the final code emission.

This PR also adds a feature `experimental_x64` to the fuzzing crate (and
the chain of dependencies down to `cranelift-codegen`) so that we can
fuzz the new x86-64 backend as well as the current one.

fuzz/Cargo.toml

@@ -17,7 +17,10 @@ target-lexicon = "0.11"
 peepmatic-fuzzing = { path = "../cranelift/peepmatic/crates/fuzzing", optional = true }
 wasmtime = { path = "../crates/wasmtime" }
 wasmtime-fuzzing = { path = "../crates/fuzzing" }
-wasm-smith = "0.1.5"
+wasm-smith = "0.1.12"
+
+[features]
+experimental_x64 = ["wasmtime-fuzzing/experimental_x64"]
 
 [[bin]]
 name = "compile"
@@ -43,6 +46,12 @@ path = "fuzz_targets/differential.rs"
 test = false
 doc = false
 
+[[bin]]
+name = "differential_wasmi"
+path = "fuzz_targets/differential_wasmi.rs"
+test = false
+doc = false
+
 [[bin]]
 name = "spectests"
 path = "fuzz_targets/spectests.rs"