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util: add a file object proxy that can notify observers

There are various places in Mercurial where we may want to
instrument low-level I/O. The use cases I can think of all
involve development-type activities like monitoring the raw
bytes passing through a file (for testing and debugging),
counting the number of I/O function calls (for performance
monitoring), and changing the behavior of I/O function calls
(e.g. simulating a failure) (to facilitate testing).

This commit invents a mechanism to wrap a file object so we
can observe activity on it. We have similar functionality in
badserverext.py. But that's a test-only extension and is pretty
specific to the HTTP server. I would like a mechanism in core
that is sufficiently generic so it can be used by multiple
consumers, including `hg debug*` commands.

The added code consists of a proxy type for file objects.
It is bound to an "observer," which receives callbacks whenever
I/O methods are called.

We also add an implementation of an observer that logs specific
I/O events. This observer will be used in an upcoming commit
to record low-level wire protocol activity.

A helper function to convert a file object into an observed
file object has also been implemented.

I don't anticipate any critical functionality in core using
these types. So I don't think explicit test coverage is
worth implementing.

Differential Revision: https://phab.mercurial-scm.org/D2462

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