Remove all global state from the caching system (#863)

* Remove all global state from the caching system

This commit is a continuation of an effort to remove usages of
`lazy_static!` and similar global state macros which can otherwise be
accomodated with passing objects around. Previously there was a global
cache system initialized per-process, but it was initialized in a bit of
a roundabout way and wasn't actually reachable from the `wasmtime` crate
itself. The changes here remove all global state, refactor many of the
internals in the cache system, and makes configuration possible through
the `wasmtime` crate.

Specifically some changes here are:

* Usage of `lazy_static!` and many `static` items in the cache module
  have all been removed.
* Global `cache_config()`, `worker()`, and `init()` functions have all
  been removed. Instead a `CacheConfig` is a "root object" which
  internally owns its worker and passing around the `CacheConfig` is
  required for cache usage.
* The `wasmtime::Config` structure has grown options to load and parse
  cache files at runtime. Currently only loading files is supported,
  although we can likely eventually support programmatically configuring
  APIs as well.
* Usage of the `spin` crate has been removed and the dependency is removed.
* The internal `errors` field of `CacheConfig` is removed, instead
  changing all relevant methods to return a `Result<()>` instead of
  storing errors internally.
* Tests have all been updated with the new interfaces and APIs.

Functionally no real change is intended here. Usage of the `wasmtime`
CLI, for example, should still enable the cache by default.

* Fix lightbeam compilation

crates/environ/tests/cache_default_config_in_memory.rs

@@ -1,10 +0,0 @@
-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!"
-    );
-}

crates/environ/tests/cache_disabled.rs

@@ -1,7 +0,0 @@
-use wasmtime_environ::cache_init;
-
-#[test]
-fn test_cache_disabled() {
-    let errors = cache_init::<&str>(false, None, None);
-    assert!(errors.is_empty(), "Failed to disable cache system");
-}

crates/environ/tests/cache_fail_calling_init_twice.rs

@@ -1,26 +0,0 @@
-use std::fs;
-use tempfile;
-use wasmtime_environ::cache_init;
-
-#[test]
-#[should_panic]
-fn test_cache_fail_calling_init_twice() {
-    let dir = tempfile::tempdir().expect("Can't create temporary directory");
-    let cache_dir = dir.path().join("cache-dir");
-    let baseline_compression_level = 5;
-
-    let config_path = dir.path().join("cache-config.toml");
-    let config_content = format!(
-        "[cache]\n\
-         enabled = true\n\
-         directory = {}\n\
-         baseline-compression-level = {}\n",
-        toml::to_string_pretty(&format!("{}", cache_dir.display())).unwrap(),
-        baseline_compression_level,
-    );
-    fs::write(&config_path, config_content).expect("Failed to write test config file");
-
-    let errors = cache_init(true, Some(&config_path), None);
-    assert!(errors.is_empty());
-    let _errors = cache_init(true, Some(&config_path), None);
-}

crates/environ/tests/cache_fail_invalid_config.rs

@@ -1,23 +0,0 @@
-use std::fs;
-use tempfile;
-use wasmtime_environ::cache_init;
-
-#[test]
-fn test_cache_fail_invalid_config() {
-    let dir = tempfile::tempdir().expect("Can't create temporary directory");
-    let baseline_compression_level = -4;
-
-    let config_path = dir.path().join("cache-config.toml");
-    let config_content = format!(
-        "[cache]\n\
-         enabled = true\n\
-         directory = {}\n\
-         baseline-compression-level = {}\n",
-        toml::to_string_pretty(&format!("{}", config_path.display())).unwrap(), // directory is a file -- incorrect!
-        baseline_compression_level,
-    );
-    fs::write(&config_path, config_content).expect("Failed to write test config file");
-
-    let errors = cache_init(true, Some(&config_path), None);
-    assert!(!errors.is_empty());
-}

crates/environ/tests/cache_fail_invalid_path_to_config.rs

@@ -1,10 +0,0 @@
-use tempfile;
-use wasmtime_environ::cache_init;
-
-#[test]
-fn test_cache_fail_invalid_path_to_config() {
-    let dir = tempfile::tempdir().expect("Can't create temporary directory");
-    let config_path = dir.path().join("cache-config.toml"); // doesn't exist
-    let errors = cache_init(true, Some(&config_path), None);
-    assert!(!errors.is_empty());
-}