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.

scripts/publish.rs

@@ -58,6 +58,7 @@ const CRATES_TO_PUBLISH: &[&str] = &[
     "wasmtime-profiling",
     "wasmtime-obj",
     "wasmtime-jit",
+    "wasmtime-cache",
     "wasmtime",
     "wasmtime-wiggle",
     "wasmtime-wasi",