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

tests/instantiate.rs

@@ -2,6 +2,7 @@ use more_asserts::assert_gt;
 use std::path::PathBuf;
 use wasmtime_environ::settings;
 use wasmtime_environ::settings::Configurable;
+use wasmtime_environ::CacheConfig;
 use wasmtime_jit::{instantiate, native, CompilationStrategy, Compiler, NullResolver};
 
 const PATH_MODULE_RS2WASM_ADD_FUNC: &str = r"tests/wat/rs2wasm-add-func.wat";
@@ -20,7 +21,8 @@ fn test_environ_translate() {
     let isa = isa_builder.finish(settings::Flags::new(flag_builder));
 
     let mut resolver = NullResolver {};
-    let mut compiler = Compiler::new(isa, CompilationStrategy::Auto);
+    let cache_config = CacheConfig::new_cache_disabled();
+    let mut compiler = Compiler::new(isa, CompilationStrategy::Auto, cache_config);
     unsafe {
         let instance = instantiate(&mut compiler, &data, &mut resolver, false);
         assert!(instance.is_ok());