add response cache with TTL + Cache-Control parsing

- New cache.rs: FIFO-eviction cache with max_bytes cap
- Cacheable: GET/HEAD only, no-store/no-cache/private/Set-Cookie reject
- TTL from Cache-Control: max-age=, or heuristics by extension (css/js/fonts/images -> 1h)
- Hook in DomainFronter::relay: check cache before network, store after 2xx
- 10 new unit tests (23 total)

src/main.rs

@@ -1,5 +1,6 @@
 #![allow(dead_code)]
 
+mod cache;
 mod cert_installer;
 mod config;
 mod domain_fronter;