Add some debug logging to fuzzers (#923)

* Add some debug logging to fuzzers

This is useful when trying to figure out what happened locally when
debugging fuzz test cases. By setting `RUST_LOG=wasmtime_fuzzing=debug`
you can get wasm files written to disk and for the API calls test case
see what API calls are being made.

* Also write out `*.wat` files

* rustfmt

* Remove return value from `log_wasm`

* Remove unused import

crates/fuzzing/Cargo.toml

@@ -6,13 +6,11 @@ name = "wasmtime-fuzzing"
 publish = false
 version = "0.9.0"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
 [dependencies]
 anyhow = "1.0.22"
 arbitrary = { version = "0.3.2", features = ["derive"] }
 binaryen = "0.10.0"
-env_logger = { version = "0.7.1", optional = true }
+env_logger = "0.7.1"
 log = "0.4.8"
 wasmparser = "0.51.0"
 wasmprinter = "0.2.0"