Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/environ/tests/cache_default_config_in_memory.rs

@@ -0,0 +1,10 @@
+use wasmtime_environ::cache_init;
+
+#[test]
+fn test_cache_default_config_in_memory() {
+    let errors = cache_init::<&str>(true, None, None);
+    assert!(
+        errors.is_empty(),
+        "This test loads config from the default location, if there's one. Make sure it's correct!"
+    );
+}