Implement mmap/munmap syscalls

[willdurand]

See: https://man7.org/linux/man-pages/man2/mmap.2.html

We'll need dynamic memory allocation sooner rather than later in userland so let's start by implementing mmap and munmap syscalls. I think we might not want to support all options, we should try to achieve the following goal(s) instead:

1. in a userland program, it should be possible to allocate some memory dynamically
2. in a userland program, it should be possible to de-allocate (free) memory
3. ideally, we should not be able to share memory between the userland programs