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/build.rs

@@ -0,0 +1,10 @@
+use std::process::Command;
+use std::str;
+
+fn main() {
+    let git_rev = match Command::new("git").args(&["rev-parse", "HEAD"]).output() {
+        Ok(output) => str::from_utf8(&output.stdout).unwrap().trim().to_string(),
+        Err(_) => env!("CARGO_PKG_VERSION").to_string(),
+    };
+    println!("cargo:rustc-env=GIT_REV={}", git_rev);
+}