Implement paging

[0xf4lc0n]

Resolves #17

The MemoryManager is useful when it comes to testing or abstracting operations that require both OffsetPageTable and BootInfoFrameAllocator.

But registering it as singleton hidden behind Once<Mutex<..>> may be not very convenient.

In future we will need MemoryManager inside alloc module or any other module that handles heap allocation.

Saying so there will be no reason to use MemoryManager outside of alloc module. So we can pass MemoryManager directly to that module and provide Once<Mutex<MemoryManager>> only for test purposes (by using #[cfg] attribute).

Feel free to provide your feedback.

[Vie0n]

LGTM