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/src/worker/tests/system_time_stub.rs

@@ -0,0 +1,29 @@
+use lazy_static::lazy_static;
+use std::time::{Duration, SystemTime, SystemTimeError};
+
+lazy_static! {
+    pub static ref NOW: SystemTime = SystemTime::now(); // no need for RefCell and set_now() for now
+}
+
+#[derive(PartialOrd, PartialEq, Ord, Eq)]
+pub struct SystemTimeStub(SystemTime);
+
+impl SystemTimeStub {
+    pub fn now() -> Self {
+        Self(*NOW)
+    }
+
+    pub fn checked_add(&self, duration: Duration) -> Option<Self> {
+        self.0.checked_add(duration).map(|t| t.into())
+    }
+
+    pub fn duration_since(&self, earlier: SystemTime) -> Result<Duration, SystemTimeError> {
+        self.0.duration_since(earlier)
+    }
+}
+
+impl From<SystemTime> for SystemTimeStub {
+    fn from(time: SystemTime) -> Self {
+        Self(time)
+    }
+}