Add basic userspace shell

[vinc]

Now that we have a basic userspace allocator (#544) we can add a basic userspace shell.

[vinc]

We have two issues with the userspace allocator:

~
> exec

> hello
DEBUG: heap bottom: 0x0000444445939808
DEBUG: ptr:         0x0000444445939808
DEBUG: heap top:    0x0000444445bb9404
DEBUG: heap bottom: 0x0000444445939808
DEBUG: ptr:         0x0000444445939808
DEBUG: heap top:    0x0000444445bb9404
DEBUG: heap bottom: 0x0000444445939808
DEBUG: ptr:         0x0000444444440c88
DEBUG: heap top:    0x0000444445bb9404
DEBUG: Could not free 0x0000444444440c88
Hello, World!
DEBUG: heap bottom: 0x0000444445939808
DEBUG: ptr:         0x0000444445939818
DEBUG: heap top:    0x0000444445bb9404
DEBUG: panicked at 'Freed node aliases existing hole! Bad free?', /home/v/.cargo/registry/src/index.crates.io-6f17d22bba15001f/linked_list_allocator-0.10.5/src/hole.rs:535:17

One when ptr points to an address outside of the process heap, and another one that I don't understand yet.

For now we can just disable freeing memory from the userspace allocator.

[vinc]

We can now execute binaries from userspace:

~
> exec

> hello
Hello, World!

> exec

> hello
Hello, World!

> quit

> quit

~
>

[vinc]

The last commit encountered another bug though:

~
> exec

> hello
Hello, World!

> hello Alice
DEBUG: Could not map Page[4KiB](0x444444445000) to PhysFrame[4KiB](0x14ad000)
Error: Could not allocate address 0x444444445630

[vinc]

Actually we get the same error if we call hello twice, and nothing is printed but no error if we call hello Alice directly.

[vinc]

Screenshot of running hello from the shell, then from exec running from the shell, then from a second exec running from the first exec:

[vinc]

More work is needed to remove the userspace allocator bugs but we can merge this PR now without including hello and exec in the default installation until the bugs are fixed.