Create a new wasmtime-fuzzing crate

This crate is intended to hold all of our various test case generators and
oracles. The fuzz targets we have at `wasmtime/fuzz/fuzz_targets/*` will
eventually be ~one-liner glue code calling into this crate.

Part of #611

crates/fuzzing/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+authors = ["The Wasmtime Project Developers"]
+description = "Fuzzing infrastructure for Wasmtime"
+edition = "2018"
+name = "wasmtime-fuzzing"
+publish = false
+version = "0.1.0"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]