Add a spec test fuzzer for Config (#1509)

* Add a spec test fuzzer for Config

This commit adds a new fuzzer which is intended to run on oss-fuzz. This
fuzzer creates and arbitrary `Config` which *should* pass spec tests and
then asserts that it does so. The goal here is to weed out any
accidental bugs in global configuration which could cause
non-spec-compliant behavior.

* Move implementation to `fuzzing` crate

fuzz/Cargo.toml

@@ -50,5 +50,11 @@ test = false
 doc = false
 required-features = ['binaryen']
 
+[[bin]]
+name = "spectests"
+path = "fuzz_targets/spectests.rs"
+test = false
+doc = false
+
 [features]
 binaryen = ['wasmtime-fuzzing/binaryen']