Add a general key-value caching framework

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/cache.h

@@ -0,0 +1,14 @@
+#pragma once
+
+struct cache;
+
+typedef void *(*cache_getter_t)(void *user_data, const char *key);
+typedef void (*cache_free_t)(void *user_data, void *data);
+struct cache *new_cache(void *user_data, cache_getter_t getter, cache_free_t f);
+
+void *cache_get(struct cache *, const char *key);
+void cache_invalidate(struct cache *, const char *key);
+void cache_invalidate_all(struct cache *);
+
+/// Returns the user data passed to `new_cache`
+void *cache_free(struct cache *);