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

src/lib.rs

@@ -29,7 +29,7 @@ pub mod commands;
 use anyhow::{bail, Result};
 use std::path::PathBuf;
 use structopt::StructOpt;
-use wasmtime::Strategy;
+use wasmtime::{Config, Strategy};
 
 fn pick_compilation_strategy(cranelift: bool, lightbeam: bool) -> Result<Strategy> {
     Ok(match (lightbeam, cranelift) {
@@ -101,3 +101,20 @@ struct CommonOptions {
     #[structopt(short = "O", long)]
     optimize: bool,
 }
+
+impl CommonOptions {
+    fn configure_cache(&self, config: &mut Config) -> Result<()> {
+        if self.disable_cache {
+            return Ok(());
+        }
+        match &self.config {
+            Some(path) => {
+                config.cache_config_load(path)?;
+            }
+            None => {
+                config.cache_config_load_default()?;
+            }
+        }
+        Ok(())
+    }
+}