Making caching support optional in Wasmtime (#2119)

This commit moves all of the caching support that currently lives in
`wasmtime-environ` into a `wasmtime-cache` crate and makes it optional. The
goal here is to slim down the `wasmtime-environ` crate and clearly separate
boundaries where caching is a standalone and optional feature, not intertwined
with other crates.

crates/environ/src/lib.rs

@@ -33,15 +33,11 @@ mod module_environ;
 mod tunables;
 mod vmoffsets;
 
-mod cache;
-
 pub mod cranelift;
 #[cfg(feature = "lightbeam")]
 pub mod lightbeam;
 
 pub use crate::address_map::*;
-pub use crate::cache::create_new_config as cache_create_new_config;
-pub use crate::cache::{CacheConfig, ModuleCacheEntry};
 pub use crate::compilation::*;
 pub use crate::cranelift::Cranelift;
 pub use crate::data_structures::*;