os: operating system features and the local filesystem

The os module provides access to resources from the host operating system,
particularly to the filesystem and standard I/O. [[cwd]] provides an
implementation of [[fs::fs]] which is backed by the host filesystem, and this
module provides equivalents of various [[fs::]] functions which infer the
current working directory as the fs parameter. This module also provides various
non-filesystem related features from the host, such as the hostname, environment
variables, and so on.