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

crates/fuzzing/Cargo.toml

@@ -15,7 +15,8 @@ log = "0.4.8"
 rayon = "1.2.1"
 wasmparser = "0.51.2"
 wasmprinter = "0.2.1"
-wasmtime = { path = "../api", version = "0.15.0" }
+wasmtime = { path = "../api" }
+wasmtime-wast = { path = "../wast" }
 
 [dev-dependencies]
 wat = "1.0.10"