General management of file descriptors, and support for
standard file descriptors (stdin/stdout/stderr).
File and file system access
This is an incomplete implementation of mmap/munmap which is restricted in order to be
implementable on top of the existing memory system. The point of these function as-written is
to allow memory allocators written entirely in Rust to be executed by Miri. This implementation
does not support other uses of mmap such as file mappings.
This implements βanonymousβ sockets, that do not correspond to anything on the host system and
are entirely implemented inside Miri.
We also use the same infrastructure to implement unnamed pipes.