Split puffin-cache into Puffin-specific and generic utilities (#728)

This crate started off as generic caching utilities, but we started
adding a lot of Puffin-specific stuff (like the cache buckets
abstraction that knows about Git vs. direct URL vs. indexes and so on).
This PR moves the generic stuff into a new `cache-key` crate.

crates/cache-key/src/digest.rs

@@ -0,0 +1,22 @@
+use std::hash::Hasher;
+
+use crate::cache_key::{CacheKey, CacheKeyHasher};
+
+/// Compute a hex string hash of a `CacheKey` object.
+///
+/// The value returned by [`digest`] should be stable across releases and platforms.
+pub fn digest<H: CacheKey>(hashable: &H) -> String {
+    to_hex(cache_key_u64(hashable))
+}
+
+/// Convert a u64 to a hex string.
+fn to_hex(num: u64) -> String {
+    hex::encode(num.to_le_bytes())
+}
+
+/// Compute a u64 hash of a [`CacheKey`] object.
+fn cache_key_u64<H: CacheKey>(hashable: &H) -> u64 {
+    let mut hasher = CacheKeyHasher::new();
+    hashable.cache_key(&mut hasher);
+    hasher.finish()
+}