Adhere to Rust naming conventions

[virtualritz]

See Rust API Guidelines on naming.

LRUCache -> LruCache
TinyLFU -> TinyLfu
SampledLFU -> SampledLfu
WTinyLFUCache -> WtinyLfuCache

Also note the inconsitencies:

• Lru is used as a prefix but Lfu is used a postfix (mostly).
• WTinyLFUCache and LRUCache have a Cache` postfix, the other caches have not.

I suggest:

LruCache
LfuCacheTiny
LfuCacheSampled
LfuCacheTinyWindow

or:

Lru
LfuTiny
LfuSampled
LfuTinyWindow

Also there are getters starting with get_, notably:

get_mru/_mut]() -> mru()/mru_mut()
get_lru/_mut]() -> lru()/lru_mut()
get_max_cost() -> max_cost()

Aliases/inline methods with deprecation warnings can be added to not break dependents.