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/cache/Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "wasmtime-cache"
+version = "0.19.0"
+authors = ["The Wasmtime Project Developers"]
+description = "Support for automatic module caching with Wasmtime"
+license = "Apache-2.0 WITH LLVM-exception"
+repository = "https://github.com/bytecodealliance/wasmtime"
+documentation = "https://docs.rs/wasmtime-cache/"
+edition = "2018"
+
+[dependencies]
+anyhow = "1.0"
+base64 = "0.12.0"
+bincode = "1.1.4"
+directories = "2.0.1"
+file-per-thread-logger = "0.1.1"
+log = { version = "0.4.8", default-features = false }
+serde = { version = "1.0.94", features = ["derive"] }
+sha2 = "0.8.0"
+toml = "0.5.5"
+zstd = "0.5"
+
+[target.'cfg(target_os = "windows")'.dependencies]
+winapi = "0.3.7"
+
+[target.'cfg(not(target_os = "windows"))'.dependencies]
+errno = "0.2.4"
+libc = "0.2.60"
+
+[dev-dependencies]
+filetime = "0.2.7"
+lazy_static = "1.3.0"
+more-asserts = "0.2.1"
+pretty_env_logger = "0.4.0"
+tempfile = "3"