ContainsOrAdd functions

[Tehhs]

PR should hopefully address #17

Changes

• Adds ContainsOrAdd(key, value) (contains, evicted) in an attempt to match hashicorps ContainsOrAdd function
• Adds similar function ContainsOrSet(key, value, ttl) (contains) to V1 & V2 as they don't really have a concept of "Add" which returns a boolean if evicted like V3. Not sure about this part, let me know.
• Unit tests

Testing

All unit tests pass in V1, V2 & V3.

[paskal]

Thanks for the contribution! The overall approach is good — atomic check-and-add without external locking is a useful feature. A few things need to be addressed:

1. Expired item check missing — In all versions, ContainsOrSet/ContainsOrAdd checks if _, ok := c.items[key]; ok but doesn't verify whether the item is expired. This means calling the method on an expired key would incorrectly return "contains=true" and skip the add, even though a subsequent Get() would return not-found. The check should also verify expiration, e.g.:

   if ent, ok := c.items[key]; ok {
       if !time.Now().After(ent.Value.(*cacheItem[K, V]).expiresAt) {
           return true, false
       }
   }

2. Test naming inconsistency — The test is named TestCacheContainsOrAdd in all versions, but v1 and v2 test ContainsOrSet. The test names should match the method names being tested (TestCacheContainsOrSet for v1/v2).

3. Missing test cases:

   • No test for expired key behaviour (calling the method on a key that exists but is expired — should treat it as not-present and add the new value)
   • No test for TTL override in ContainsOrSet (v1/v2)
   • No concurrency test for the new method

4. v1 interface change — Adding ContainsOrSet to the Cache interface in v1 is a breaking change for anyone who implements the interface externally. I would rather avoid it, maybe let's just add it to v3 if there is no way around it.

The naming split (v1/v2 use ContainsOrSet with TTL param, v3 uses ContainsOrAdd matching hashicorp's signature) makes sense given the API differences between versions — v3 has Add() while v1/v2 only have Set().