Making caching support optional in Wasmtime (#2119)

This commit moves all of the caching support that currently lives in
`wasmtime-environ` into a `wasmtime-cache` crate and makes it optional. The
goal here is to slim down the `wasmtime-environ` crate and clearly separate
boundaries where caching is a standalone and optional feature, not intertwined
with other crates.

crates/cache/tests/cache_write_default_config.rs

@@ -0,0 +1,12 @@
+use wasmtime_cache::create_new_config;
+
+#[test]
+fn test_cache_write_default_config() {
+    let dir = tempfile::tempdir().expect("Can't create temporary directory");
+    let config_path = dir.path().join("cache-config.toml");
+
+    let result = create_new_config(Some(&config_path));
+    assert!(result.is_ok());
+    assert!(config_path.exists());
+    assert_eq!(config_path, result.unwrap());
+}