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/src/lib.rs

@@ -43,7 +43,8 @@ pub use crate::address_map::{
     FunctionAddressMap, InstructionAddressMap, ModuleAddressMap, ModuleMemoryOffset,
     ModuleVmctxInfo, ValueLabelsRanges,
 };
-pub use crate::cache::{create_new_config as cache_create_new_config, init as cache_init};
+pub use crate::cache::create_new_config as cache_create_new_config;
+pub use crate::cache::CacheConfig;
 pub use crate::compilation::{
     Compilation, CompileError, CompiledFunction, CompiledFunctionUnwindInfo,
     CompiledFunctionUnwindInfoReloc, Compiler, Relocation, RelocationTarget, Relocations,