Segregate kernel/memory.hpp into meaningful parts

[mazunki]

The old kernel/memory.hpp had several responsibilities, and albeit not being super long it felt hard to read. It comprised at least two fairly different parts:

• Setting up a mapping from virtual memory to physical pages (now mem/vmap.hpp)
• Setting up the allocator for the heap (now mem/alloc.hpp)

Furthermore, I've also created mem/flags.hpp since it doesn't quite fit either of them entirely, and it facilitates plans going forward.

Currently, we're assuming architecture-specific/hardware protection flags match IncludeOS-allocation flags. While this is currently the case, I do intend to change this in the future for the sake of type safety. src/arch/x86_64/paging.cpp uses os::mem::Access referring to hardware flags, while src/kernel/multiboot.cpp or src/kernel/elf.cpp refers to IncludeOS semantics. I intend to create a per-architecture translation mapping for this (constexpr, so it won't affect any performance, of course).

The responsibility separation between mem/vmap and mem/alloc seems to be represented in the usages across the code too (most of the results are non-overlapping):

$ rg mem/vmap.hpp | wc -l
18
$ rg mem/alloc.hpp | wc -l
14

This PR essentially makes #2329 obsolete.