Fuzz using precompiled modules on CI (#3788)

In working on #3787 I see now that our coverage of loading precompiled
files specifically is somewhat lacking, so this adds a config option to
the fuzzers where, if enabled, will round-trip all compiled modules
through the filesystem to test out the mmapped-file case.

crates/fuzzing/Cargo.toml

@@ -13,6 +13,7 @@ arbitrary = { version = "1.0.0", features = ["derive"] }
 env_logger = "0.8.1"
 log = "0.4.8"
 rayon = "1.2.1"
+tempfile = "3.3.0"
 wasmparser = "0.82"
 wasmprinter = "0.2.32"
 wasmtime = { path = "../wasmtime" }