Moved memory-management crates into kernel/memory

[NathanRoyer]

This moves the following crates:

• block_allocator
• entryflags_x86_64
• frame_allocator
• memory_initialization
• memory_structs
• memory_x86_64
• page_allocator
• page_table_entry

into theseus/kernel/memory, similarly to what was done to ACPI-related crates.

With this PR, there are less crates in theseus/kernel/.

Additionally, most non-memory crates now use re-exports from the memory supercrate, which makes it easier to know what re-exports other crates actually depend on, from these crates. There is one exception: I left the direct import from nano_core to memory/memory_initialization because removing that dependency looks more difficult than what I did in this PR; removing that dependency can have its own PR, imo.

[NathanRoyer]

Note: all file modifications with this icon: are files that were simply moved without internal modification. I hope this simplifies your review.

[kevinaboos]

Thanks, this looks pretty good. Also a step forwards in the right direction re: #51.

Currently there is a CI build error, but I'm going to wait until all the current memory subsystem changes are merged to address this anyway.

[kevinaboos]

alrighty, all the memory subsystem changes have been merged in now, so we can accept this PR whenever you're able to merge in the latest changes from theseus_main and address the failing CI checks. :+1: