Add get_or_insert_with method

[tatsuya6502]

This PR adds the get_or_insert_with and get_or_try_insert_with methods to sync and future caches in Moka. These methods ensure the key exists in the cache by inserting the return value from the given closure/future if the key does not exist. They also guarantee the closure/future is evaluated only once even if they are called from multiple threads/async tasks for the same key at the same time.

This feature was requested by #15.

Added

• (API) Add get_or_insert_with and get_or_try_insert_with methods to the following cache implementations in Moka:
  • sync::Cache
  • sync::SegmentedCache
  • future::Cache
• Add internal sync::value_initializer and future::value_initializer modules, containing cht::HashMap for waiters.

---

Fixes #15

[tatsuya6502]

Merging.

[ben-manes]

I don't know rust conventions, but Java's names in Map are quite nice.

• computeIfAbsent(key, key -> value)
• computeIfPresent(key, (key, oldValue) -> value)
• compute(key, (key, oldValue) -> value
• merge(key, value, (key, oldValue) -> newValue)

These are performed atomically and if null is returned then the mapping is removed. The names are nice as succinct and easy to understand. The three compute methods are very useful for advanced cases.

Caffeine's corre apis uses get for automatic loading, getIfPresent without, and offers an asMap() view for richer functionality (like compute or entry iterators). Since we highly prefer users to delegate to the cache for loading, we take the desirable get name and penalize the longer name getIfAbsent to dissuade usage. A lot of random functionality, like conditional entry removal or policy inspection, is hidden away under nested objects (asMap(), policy()) to provide the features while keeping the core interface simple to reduce the conceptual weight for learning a new library.

I don't know if any of that makes sense idiomatically for Rust. If so, you might consider reviewing our APIs for ideas before yours are finalized.