Expire logic

Features:

• Add expire(const key_type&) method, that expires an entry right then.

Bug fixes:

• Fix incorrect forwarding.
• Fix use-after-free bug.

Instrumentation

Added instrumentation to the cache.

Instrumentation requires https://github.com/nahratzah/instrumentation to be installed.
This is an optional component.
If it is not installed, the instrumentation methods won't do anything (except report an exception or refuse to compile, if you try to use them).

Initial release

This is the first version, so everything is new! \o/