io: high-level I/O abstraction and low-level file I/O

The io module provides input and output (I/O) functionality for Hare programs,
such as reading from or writing to files. The I/O module is not generally
responsible for provisioning the I/O objects themselves; see modules like
[[os::]] and [[net::]] for this purpose.

I/O operations such as [[read]] or [[write]] accept an I/O handle, [[handle]],
to specify the object of the I/O operation. This type is a tagged union of
[[file]] and *[[stream]]. Most programmers should prefer to use [[handle]]
unless they specifically require the special semantics of one of its subtypes.

The [[file]] type provides access to an object, usually a file descriptor, which
is provided by the host operating system. It represents objects such as a file
on disk, an open network connection, and so on. The use of [[file]] is generally
required when working with host I/O, such as with [[unix::poll::]].

The [[stream]] type is an abstraction that allows Hare programs to implement
their own I/O objects by providing implementations of [[read]], [[write]], and
other functions, for an [[handle]]. Several standard library modules offer
implementations of [[stream]] for one reason or another, such as [[bufio::]].
Additionally, the io module provides some useful general-purpose I/O streams,
such as [[tee]].
